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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Weldability Diagram for the Ultrasonic Welding of Thermoplastics in Far-Field
Ultrasonic welding has proven itself to be an efficient way of joining thermoplastic parts. It is accomplished by applying high frequency, small amplitude vibrations to the pieces to be joined together. To make satisfactory products, the material, the horn, and the process parameters determine the weldability of a specific system. The concept of welding areas on the critical plane can be extended from injection molding to the ultrasonic welding system of thermoplastics. This research is devoted to the development of a proper weldability diagram for ultrasonic welding in far-field. The first part of this study is to identify the relative importance of process parameters. The results show that weld time and amplitude of vibration are the two factors which most significantly affect the ultrasonic welding process. The second part of this study defines the weldability based on critical parameters. The application of the weldability concept is demonstrated in selecting the optimum part and energy director geometry. In addition, the temperature variations at the joint interface were recorded to better understand the welding process. These weldability studies are intended to give first guidelines for system optimization.
Optimizing the Joint Strength of Ultrasonically Welded Thermoplastics in Near-Field
Ultrasonic welding of thermoplastics has become an important process in industry because of its relatively low cost and high quality joints. However, the optimization of this technique has been essentially based on a trial-and-error process. In this report, an L'18 experimental matrix design based on the Taguchi method was conducted to optimize the joint strength of ultrasonically welded thermoplastics. For the factors selected in the main experiments, weld time and amplitude of vibration were found to be the principal factors affecting the joint property of ultrasonically welded thermoplastics. A weldability diagram was proposed based on the statistical results to give first guidelines for system optimization. In addition, amorphous polymers required less energy to be successfully welded than semi-crystalline polymers. Semi-circular energy directors were found to bond parts of highest strengths.
Influence of Applied Pressure on Erucamide Diffusion in LLDPE Films
There are many factors that influence the diffusion of an additive to the surface of a polymer film. One of the factors that is not often studied is applied pressure, which is an important factor because the pressure exerted on film near the inside of a roll may be significantly different than the pressure exerted on the film near the outside of a roll. We have demonstrated previously that a higher applied pressure tends to hinder additive diffusion. This paper studies additive accumulation at film/film interfaces as a function of applied pressure. The results indicate that a decrease in additive at the surface coincides with an increase in applied pressure.
Developing a Surface Texture with Film Insert Molding (FIM)
The surface texture of an Injection molded part in the film insert molding process is affected by the Molding injection machine parameters. Other variables include the molded material, the film texture and thickness, the quality of the applique, and the mold finish. In this study three different films, using two thicknesses were processed. The injection speed and mold temperatures were varied. Three different resins were evaluated. A 102 mm x 152 mm (4 in x 6 in) part with six different textures was used to determine the gloss and roughness of the finished part with each set up. The goal was to determine the affects on the finish of the final part in the FIM process.
PEN-Poly(ether imide) Blends: Melt Rheology and Stress Relaxation in Semi-Crystalline Films
Past studies on the stress relaxation of biaxially oriented polyester films are extended to a blend of poly(ethylene-2,6-naphthalate) (PEN) and poly(ether imide) (PEI) at a composition of 85/15 by wt. At this composition the polymers are fully miscible and the melt viscosity and flow activation energy of the blend are dominated by the PEN component. PEI plays a more important role in controlling the sub-Tg stress relaxation response of films produced from the corresponding blend. We show that the relaxation time of a biaxially oriented, semi-crystalline film is increased upon addition of PEI to the PEN matrix, consistent with the rise in glass transition temperature. Also, using stress relaxation data in combination with the concept of aging acceleration factors, we probe the kinetics of the physical aging process for the neat PEN and blend films over a temperature range spanning from ca. Tg - 5O°C to Tg + 10°C. Over this temperature interval both films eshibit thermorheological simplicity with respect to time-aging time and time-aging temperature superposition and the rate of physical aging peaks at ca. Tg - 20°C. However, the relative increase in the rate of aging is much lower for the blend, thus suggesting that the PEI component is well dispersed in the PEN matrix and is effectively retarding the segmental mobility of PEN.
On-Line Analysis of Stress and Birefringence Development in Simultaneous Biaxial Elongation of Poly(ethylene terephthalate)
On-line measurements of elongational stress and birefringence of poly(ethylene terephthalate) film were performed under simultaneous equal-biaxial elongation at various strain rates and temperatures. The strain hardening started to occur at a true strain of about unity, which was practically independent of drawing condition. Even though the drawing was done under the equal-biaxial condition, the PET films started to show optical anisotropy in the film plane before the strain hardening. The optical anisotropy was more significant under the drawing conditions of high strain rate and low temperature, however it only appeared in the limited range of strain and disappeared again in all the samples when strain was high enough. The starting point of the optical anisotropy corresponds to the off-plane birefringence, which is defined as the refractive index difference in the directions parallel and normal to the film plane, of about 0.03. This birefringence also corresponds to the critical point above which the stress-optical law is not applicable because of the starting of spontaneous molecular orientation. A similar spontaneous orientation behavior was also observed in the uniaxial elongation process.
Low Extractable, Low Blush, Low Color PVC with Improved Processibility for Medical Application
The present paper provides an additive system for a PVC formulation to improve processing and functional characteristics. This additive system improves melt fabrication and heat stability of the PVC (e.g., increased thermal stability for faster melt flow during extrusion without discoloration or formation of black specks; eliminates further plate-out or build-up on the processing equipment; decreased scrap and equipment down-time; permits higher use of regrind) and the desired product charateristics (e.g., low color, high clarity, low-blush, extractables and particle generation).
Temperature-Moisture-Mechanical Response of Vinyl Ester Resin and Pultruded Vinyl Ester/E-Glass Laminated Composites
Pultruded E-glass composites are gaining acceptance in the civil infrastructure market due to their low maintenance and high durability; yet polymer composites are more sensitive than metals to temperature and moisture effects. The present requirements for polymer composites are stringent, with lifetime expectancies for some applications being around 50-75 years. The goal of the present work is therefore to lay the groundwork for future accelerated aging research and analysis. In order to achieve this, the present paper outlines the quasi-static tensile response of vinyl ester polymer and pultruded vinyl ester/E-glass (cross-ply and angle-ply) composite specimens subjected to both different test temperatures as well as hygrothermal aging for different periods of time. Performance at test temperatures ranging from -50 to 95°C and moisture contents varied by saturation in water baths at 45°C and 80°C are presented. Damage induced by exposure of specimens to varied temperatures and elevated water temperature immersion during quasi-static testing are also analyzed for future durability studies.
Study of Blending Coextruded Film Reprocessed with LDPE and HDPE
Blends of five layer coextruded films: linear low density polyethylene LLDPE/tie/polyamide-6 (PA6)/tie/LLDPE and low density polyethylene (LDPE) and high density polyethylene (HDPE) were reprocessed by melt mixing. The object of this work is to study mechanical properties and processability of multicomponente blends of coextruded film reprocessed with LDPE and HDPE resins. These blends were run on an injection molding machine and the effect of the reprocessed blend content on their mechanicals properties were determined. An increase in mechanical properties as well as a reduction in flow properties was observed when increased the reprocessed blend content.
Structure-Properties of PP-EPDM Thermoplastic Elastomer: Origin of Strain Recovery
The question is why PP-EPDM TPE can shrink back from the highly stretched states at ambient temperature, even though the two-phase material consists of a ductile polymer (PP) matrix. To answer the question, we carried out the structure characterization by WAXD and TEM and the FEM analysis on deformation mechanism. The elastomeric nature was shown to originate partly from a characteristic morphology of PP crystallites; i.e. the rather fragmented crystal lamellae formed in the presence of polymer impurity (EPDM) occluded in PP matrix under high shear fields during dynamic vulcanization. That is, the PP matrix itself is expected to be less ductile and more elastomeric than neat PP. The FEM analysis revealed that the plastic deformation of PP matrix can be healed by the volumetric strain of the rubber particles with high Poisson's ratio (~0.5; very small volume change with deformation).
Computer-Aided Thermoformed Product and Process Design
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.
Heat Affected Zone Structure in Vibration Welded Polyvinylidene Fluoride and its Copolymer
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.
Control of the Melt Strength of Polypropylene in Rotational Foam Molding
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.
Processing of Polypropylene Foams in Melt Compounding Based Rotational Foam Molding
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.
Luggage Retention and Automotive Rear Seats Designed Using Finite Element Analysis Tools
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.
Co-Continuous Homopolymer/Copolymer Blends
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.
Toward a Weld-Strength Data Base for Hot-Tool Welding of Thermoplastics
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.
Development of a Dilatometer for Measurement of the PVT Properties of a Polymer/CO2 Solution Using a Foaming Extruder and a Gear Pump
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.
Effect of Supercritical CO2 and N2 on the Crystallization of Linear and Branched Propylene Resins Filled with Foaming Additives
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.
A Study on Melt Fracture of Linear and Branched Propylene/Butane Solutions with Additives
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.
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