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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Modification of Rheological Properties of LDPE for Coating Applications
The effect of shear modification and blending on the coating performance of a commercial LDPE resin was studied. Shear modification of the virgin resin as well as of its blends with another LDPE was performed in a single and in a twin screw extruder. The rheological properties of the modified materials were measured and no significant differences in their behavior under shear deformation were detected. On the other hand, changes in the entrance pressure drop and extensional viscosity were observed. These changes in extensional behavior were found to affect coating properties. Overall, neck-in improved through shear modification while it was not sensitive to blending with a higher molecular weight resin. On the other hand, blending had an effect on the draw-down speed and it may be used to achieve a balanced coating performance.
Study of Color Change Kinetics during RPVC Extrusion
Executing a color change is a routine operation for plastic processors. With on-line coloring in extrusion, the polymeric melt continuously flows shaping the product and changing its color from one color (color A) to another color (color B). At the moment of initiating the color change, there is a period of time in which the product will yield acceptable color (in color A). After some amount of time elapses, the transition phase from color A to color B will yield off-color product until the color change for color B is fully established in the system. The amount of time it takes to make this change from color A to color B depends on a variety of factors including the extruder size and type, shaping die, resin that is extruded, coloring system, and the color to be changed, whether from light to dark or vice versa. In this report, an approach for minimizing both the color-change extrusion time and off-color rejected product is presented. An extrusion simulation is applied to determine temperature - residence time -shearing correlation and the correct let down ratio for a PVC compound/pigmentation system. A Brabender mixer was used as an instrument. The information obtained was then applied for studying color change kinetics in a production environment.
Microstructure of Vibration Welded Nylon 66 Joints
Vibration welding is a useful technique for joining thermoplastics. This vibration welding study examined the microstructure of joints made of 33% glass reinforced nylon 66. Joints were made under different vibration welding conditions. The microstructural analysis techniques were: optical microscopy of weld cross-sections under polarized light, scanning electron microscopy of both weld cross-sections and fracture surfaces, and thermal gravimetric analyses near the weld line. These tests provide information on the thickness of the heat-affected zone, glass fiber orientation and level respectively. The microstructural analyses reported in this study assist in the interpretation of experimental tensile weld strengths.
Rotational Moulding of Flame Retardant Linear Low Density Polyethylene (FR-LLDPE) Compounds
Flame retardant polyethylene compounds fabricated from rotational moulding technique are finding widespread applications for Industrial applications, stadium seats, children`s toys in parks etc. In the present work , the objective was to assess rotational moulding characteristics of flame retardant -LLDPE. The end application identified for the purpose was a hollow box for packaging of explosive materials. The material currently used is wood. With increasing pressure to conserve natural resources, the present work acquires significance and provides impetus in that direction. Compounds were designed to meet V0, V1, and V2 Flammability rating as per UL 94 test protocol. FR-LLDPE compounds were prepared on Berstoff ZE-25 twin-screw extruder. Mechanical properties are determined on the injection moulded ASTM test specimens and cut specimens from rotational moulded boxes. The flammability test on rotational moulded hollow containers has been captured in a compact disk.
Effect of Crosslinking on Swelling, Transitions and Mechanical Properties of Poly(N-Isopropylacrylamide) Responsive Hydrogels
N-isopropylacrylamide (NIPA) was copolymerized with different concentrations of N, N'-methylenebisacrylamide (MBA) to obtain networks of different crosslinking densities. Semiinterpenetrating networks of the same crosslinking densities were obtained by copolymerization of NIPA and MBA in the presence of linear polyacrylamide (5 wt.%). The equilibrium swelling degree in water at 25°C varied from 7.61 to 16.8 times the original volume when the molar ratio NIPA/MBA increased from 10:1 to 100:1. The transition temperature determined by DSC was largely unaffected by the crosslinking conditions in this work. Crosslinking density obtained from dynamic shear measurements was much lower than predicted from the molar ratio of the monomer and crosslinker, which was explained by the incomplete reaction. The sol fraction varied from 12% to 30%. Thermomechanical analysis showed contraction followed by the expansion of the gels during transition. The broadening of the temperature range of the gel collapse was observed as the crosslinking density increased. Tensile strength of the 10% swollen semiinterpenetrating NIPA networks was higher (15-23 kPa) than that of the 10% swollen single NIPA networks (9-13 kPa). Elongations at break were significantly higher for semi IPN gels compared to the single ones, especially at lower crosslinking densities.
Apparent Extensional Properties of Injection Moulding Grade Polyethylenes Filled with Hydroxyapatite
The apparent extensional properties of three injection molding grade, high density polyethylenes (PE) and their composites containing 20vol% hydroxyapatite (HA) were studied using orifice pressure drop data from a capillary rheometer followed by Cogswell's analysis. The effects of temperature, molecular weight and filler have been investigated. For unfilled PE extensional viscosity was found to decrease as the molecular weight decreased or the temperature increased. The addition of HA appears to increase the extensional viscosity at low strain rates and decreases due to extension thickening at high strain rates. HA addition tended to decrease the Trouton ratios of PE at moderate to high strain rates.
Injection Molding Thermoplastics to Tolerances of One Micron
The tolerances on injection molded thermoplastic parts have grown tighter over the past several decades, and concomitantly the range of applications for thermoplastic injection molded parts have grown dramatically. Now, tolerances of one micron can be achieved, expanding the range of applications for injection molded thermoplastics even more. The tolerances on the injection molded thermoplastic parts are precipitated by the combination of the tolerances from the feedstock, the injection mold, the injection molding machine, the peripherals, the control system and the part measurement devices. So, in simple terms, each of these contributing tolerances must be minimized in order to minimize the tolerances on the injection molded part. In the specific applications herein described, which is a single part fabricated in a multi cavity mold the tolerances on the injection mold are found to be the primary contributing tolerances, the tolerances on the control system are found to be the secondary contributing tolerances, and the tolerances on the injection molding machine and the feedstock are less significant.
Weldline Strength in Products Manufactured through Vibration-Assisted Injection Molding
Weldlines, which are a common feature in many injection molded parts, are known to be inherently weak. The present experimental study focused on improving weldline strength through the addition of oscillatory vibrational energy during molding processes. Both filled and unfilled polymers were studied, and comparisons between the tensile strength of ASTM specimens made with and without vibration were made. It was found that minimal product strength increases were realized when optimized vibration-assisted molding conditions were applied. Details related to the critical vibration-assistance parameters studied and optimization processes utilized will be presented and discussed.
Preparation and Properties of Polypropylene Nano-Composites
Polypropylene (PP) nano-composites are prepared by melt intercalation in this study. Nano-clay is mixed with PP by twin screw extrusion. Maleic anhydride modified polypropylene (PP-MA) is added to enhance the dispersion of the clay in PP. Nylon 6 is also added to this PP/PP-MA/clay composite to partially replace the content of PP-MA in order to decrease the material cost. The basal spacing of the clay in the composites is measured by X-ray Diffraction (XRD). The morphology of the composites is observed by SEM. The mechanical properties of the composites are also measured. It was found that the molecular weight and MA content strongly affect the nano-structure and the properties of PP nano-composites. PP-MA with a lower molecular weight (LMW) and a high MA content can lead to good clay dispersion in PP-MA. However, it causes a decrease in the mechanical properties of PP/PP-MA/clay composites. It was also found that the addition of nylon 6 further expands the basal spacing of the clay in PP composites. However, the mechanical strength decreases by adding nylon 6 to the nano-composites.
Design and Fabrication of Polymer Microfludic Platforms for Biomedical Applications
In this paper, design and fabrication of a CD platform, where different fluidic structures are micromachined into a plastic CD, are presented. Various ways to fabricate mold inserts (traditional CNC-machining of tool steel vs. photolithography/electroplating) and microfluidic platforms (micro-embossing, thin wall injection molding, and reactive molding) based on optically clear thermoplastic and thermoset polymers (e.g. optical quality polycarbonate, PMMA, PDMS) are introduced. The effects of mold inserts, molding methods, and feature size on replication accuracy and 'molded-in' stresses are also discussed.
Effect of Co-Promoter and Secondary Monomer on Shrinkage Control of Unsaturated Polyester Resins Cured at Low Temperatures
The presence of low profile additives (LPAs) can reduce the shrinkage of unsaturated polyester (UP)/styrene (St) resins cured at low temperatures, but the final shrinkage is still quite high. In this study, secondary monomers (e.g. divinylbenzene (DVB) and trimethylopropane trimethacrylate (TMPTMA)) and co-promoter 2,4-Pentandione (2,4-P) were added into the UP/St/LPA system to investigate their effect on shrinkage control of resins cured at low temperatures. Dilatometery results showed that the addition of both TMPTMA and 2,4-P resulted in earlier volume expansion during curing and in turn; better shrinkage control. The phase separation, reaction kinetics and viscosity changes in the LPA-rich and UP-rich phase during curing were investigated. Results revealed that the increased reaction rate in the LPA-rich phase led to an earlier formation of microvoids and, consequently, less volume shrinkage of the cured resin.
Effect of Resin Chemistry on Curing of Unsaturated Polyester and Vinylester Resins
Unsaturated polyester and vinylester resins are the two major thermoset resins used in low temperature composite manufacturing processes, such as the Seemann Composite Resin Infusion Molding Process (SCRIMP). Understanding the reaction kinetics and network formation can be critical to SCRIMP. A series of well-defined polyester resins are used to study the effect of resin chemistry on the reaction kinetics. The effects of styrene concentration and temperature on the cure kinetics of styrene/vinylester systems are also studied. The glass transition temperature of polyester/vinylester resins cured at different temperatures is measured and used to monitor the final conversion change. A mechanistic kinetic model is proposed to simulate both the reaction kinetics and the final conversions of various resins in low temperature copolymerization. The results may shed light on resin modification and process innovation.
Supercritical Carbon Dioxide Assisted Polymer Blending in Twin-Screw Extrusion: Phase Inversion, Morphology, and Mechanical Properties
Supercritical carbon dioxide (scCO2) was added during compounding of polystyrene (PS)/poly(ethylene-methyl acrylate) copolymer (SP 2207) and poly(methyl methacrylate) (PMMA)/SP 2207 blends and the resulting phase inversion, morphology, and mechanical properties were studied. SP 2207 is a 80 wt.% ethylene-20% methyl acrylate copolymer used to increase the toughness of polystyrene and PMMA. Carbon dioxide was added at 2.0 wt.% based on polymer melt flow rates. Carbon dioxide injected into the extruder sped up the phase inversion of the two components by reducing the glass transition temperature of PMMA and PS. This allowed for an enhanced dispersion of the rubber phase after phase inversion. An earlier extruder location at which phase inversion occurred along with a reduced viscosity ratio greatly improved the dispersion of the rubber copolymer in the polystyrene and PMMA matrix. An increase in impact strength and a reduction in the flexural modulus of blends prepared with CO2 were measured, indicating that CO2 aided in dispersing the rubber into the brittle matrices of polystyrene and PMMA. Reprocessing did not result in a loss of morphology for PMMA/SP 2207 blends.
A Novel Resin Design for Styrenic Resin-Based Room Temperature Vacuum Infusion Resin Transfer Molding-Scrimp
For low temperature composite manufacturing processes, a major concern for the fabricators is how to design and control the mold filling and curing time. Curing agents like inhibitors or retarders are needed to prevent premature gel and provide a sufficiently long time to complete mold filling. However, the addition of inhibitors or retarders tends to result in a slow mold curing rate and low resin conversion. In this study, a chelating agent, 2,4-pentanedione (2,4-P), is used to manipulate the resin gelation and curing. 2,4-P can interact with promoters (i.e., metal compounds such as cobalt carboxylates) and affect the catalytic activity of the promoter to decompose the initiators, by either retarding or promoting the resin system depending on the initiator and resin system used. It was found that the function of 2,4-P as a retarder or co-promoter for the co-polymerization of styrenic resins depends greatly on the acidity of the resin system used. Based on this observation, a series of SCRIMP molding experiments using a novel resin design was carried out. This design allows 2,4-P to serve as a retarder during mold filling to achieve a long gel time; it then becomes a co-promoter during curing to increase the curing rate and conversion when the acidity of the resin changes.
Instabilities in Startup Flows of Polyolefin Melts in Axisymmetric Contraction Geometries
A flow visualization cell with an abrupt entry pseudo-axisymmetric geometry has been used in the study of a low density polyethylene melt flow. The flow cell was mounted on a Davis Standard Betol BK38 single screw extruder. Birefringence and imaging velocimetry techniques have been used to observe changes in the stress and velocity fields in the melt at various processing conditions. Flow development of the melt was observed using pigment tracers. Start-up and steady state flow regimes are investigated. A transient instability occurred on start-up flows. The time stabilization of the flow increased with dwell time between extrusion trials. The instability was observed as oscillatory motion of the stress birefringence pattern and fluctuations in measured pressure.
Flow Visualisation of Polymer Melt Contraction Flows for Validation of Numerical Simulations
Full field velocity measurements in planar contraction geometries are reported - these having been evaluated using the techniques of particle image and particle streak velocimetry for HPDE and LDPE melt flows. Measurements were made in a specially designed flow cell that enables laser sheet lighting to be used to illuminate planes of the polymer at precise locations across the die, mounted on a Betol BK38 extruder. Stress measurements obtained by flow birefringence complement the velocity field information. Steady state and start up flow regimes were investigated with results presented in a form suitable for direct comparison with ongoing numerical simulations.
Combined Process Variables and Process Energy Monitoring for Injection Moulding
Good injection moulding machine control is a necessary requirement for control of the process, however there is an acknowledged lack of process understanding, related in turn to a lack of understanding of the polymer under process conditions which inhibits the development of standardised route to process control. In our laboratory, specific pressure indices in an identified low noise region of the primary injection stage of injection moulding have been found to provide a sensitive indicator of changes in a polymer, including batch to batch changes and process-induced changes, which in turn allows meaningful Statistical Process Control to be undertaken. Growing concern for environmental issues, including international standards agreements such as ISO14001, demonstrate a clear requirement to conserve energy for both environmental and cost issues. Detailed energy measurements on injection moulding machines both in the laboratory and in industry demonstrate the potential of process energy measurements as an aid to the development of a systematic management approach to the environmental concerns of an organization. Laboratory DOE studies allow a further insight into the influence of a variety of machine variable settings on the total energy consumption. We are currently in the process of combining both process variable and process energy measurements, to provide processors with the richest level of process information.
Development of Constitutive Equations for Solid Phase Deformation of Polymers with Time-Varying Temperature
The large deformation, nonlinear viscoelastic behaviour of polymers has been explored at elevated temperatures. Experiments consist of uniaxial tensile stress relaxation experiments. For isothermal experiments, the results can be represented using models consisting of Eyring processes and elastic networks. Experiments have also been carried out on specimens subject to controlled cooling, to simulate the development of the room- temperature mechanical properties of processed products. Progress in extending the Eyring-based theories to these conditions is reported.
Injection Moulding Machine Performance Inter-Comparisons
An extensive study has been carried out to compare the performance of four highly instrumented injection moulding machines, using the same polymer (HDPE) and tensile test specimen mould. A 50 tonne servo-electric machine, a 30 tonne servo-electric machine, a 75 tonne servo-hydraulic machine and a 60 tonne proportional- hydraulic machine were studied, to compare accuracy, repeatability and process control. In each case, nozzle melt pressure and temperature, tool temperature and Viscosity Index (a primary injection pressure integral) were monitored at 50Hz during each cycle. A Design of Experiments methodology together with long production runs were employed to compare machine performance at identical moulding conditions across the full process window. Comparable product quality was observed between the four machines but significant differences in process stability and control were highlighted by the in- process measurements.
Extrusion Temperature Field Measurement: A 'Soft Sensor' Approach
A range of temperature measurement techniques have been employed in single and twin screw extrusion in conjunction with CFD modelling in an attempt to build up an accurate picture of the temperature field across a flowing melt. A motorised traversing thermocouple provided a temperature profile at a range of extrusion conditions, with corrections made for conduction losses along its length. A non-intrusive infra red sensor was used to measure melt temperature several millimetres into the melt flow and non-intrusive high temperature ultrasonic sensors gave an indication of bulk temperature across the flow. A commercial 3D computational fluid dynamics package was used to model the melt flow and provide predicted thermal plots. A 'soft sensor approach', combining measurement routes and modelling, enables a detailed temperature field to be obtained in extrusion. This improving understanding is being used in conjunction with commercial 'DualTherm' barrel temperature control, with a view to enhanced control of polymer melt temperatures during extrusion.
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