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.
The foaming behaviours of blends of linear and branched propylene polymers are presented in this paper. The effects of increasing the amount of branched propylene polymer materials on the expansion ratio and cell density of extruded foam were examined. In foam processing of linear polypropylene resins, the quick loss of blowing agent from the foam, promoted by cell coalescence, causes foam contraction, and results in low expansion. On the other hand, the branched propylene polymer resins are well known to retard cell coalescence, to increase the expansion ratio, and to increase the processing window. Due to the high cost of branched resin and in order to design foam properties, the use of blends of linear and branched propylene polymers has been of interest to the foam industry. This paper presents the changes in the processibility and foam qualities as a function of the amount of the branched resin. The foam morphologies of propylene polymer blends at various processing temperatures were investigated using a tandem foam extrusion system, and their volume expansion and cell nucleation behaviours were compared. As expected, increasing the percentage of branched resin in the blends promoted the volume expansion and cell density that resulted in an increase in the processing window for the maximum achievable volume expansion, and a finer cell structure.
In thermoplastic foam processing using physical blowing agents, the degree of swelling that occurs is determined by the amount of the blowing agent dissolved in the polymer. From an analytical point of view, the swelling that occurs due to gas dissolution can be determined by obtaining the pressure-specific volume-temperature (PVT) data of the polymer/gas solution. The basic principle involved in the measurement of PVT properties of polymer/gas solutions is to measure the specific volume of the solution by determining the mass and volume flow rates of the polymer/gas solution at different temperatures and pressures. This paper presents a dilatometer based on a foaming extruder with a new degassing oven to facilitate the measurement of the mass flow rate of polymer/gas solutions from the foaming extruder. The degassing oven allowed us to completely remove traces of a blowing agent with a low diffusivity from a foam sample while simultaneously measuring the mass of the foam. A series of parametric experiments revealed significant swelling of the propylene due to the dissolved butane. At 15 wt% of butane the swelling of the propylene materials was observed to be as much as 20%. With identical butane concentrations, it was also observed that the swelling of linear propylene was generally greater than the swelling of branched propylene under similar temperature and pressure conditions. The effects of temperature and pressure on the specific volume were also observed to be different for the pure polymer and the polymer/gas solution.
For standard injection molding a granule length of 10 - 12 mm is used in long glass fibers applications to achieve an optimum fiber length distribution in the part. The resulting fiber length in the part is limited to approximately 3 - 5 mm. To increase the fiber length in the molded part it is necessary to directly compound the glassfibers into the polymer on the injection molding machine. The presentation describes a very cost effective way to produce injection molded parts with long glass fibers on a system where a discontinuous running twin screw extruder fed with glass rovings is directly mounted on a two stage injection unit. The advantages of this configuration compared to a continuously running twin screw extruder and discontinuously running single screw extruder mounted on a two stage injection unit were evaluated and are described in depth.
The rotational foam molding process is suitable for fabricating rotational moldings intended for applications that require their internal volume to be entirely or partially occupied with a cellular structure. Whether the outcome of this process will result in articles having a fully foamed core or just a foamed layer with a given thickness depends on the volume expansion ratio for which the foamable resin has been formulated and the shot size the mold has been charged with. Although the amount of chemical blowing agent (CBA) that should be introduced into the polymer in order to prepare a foamable resin that would be suitable for satisfying the required expansion characteristics can be theoretically calculated, in practice, due to various reasons, a portion of the generated blowing gasses during the rotational foam molding cycle is often inevitably lost via the mold's vent(s). As a consequence of this unaccounted loss of blowing gas, a discrepancy exists between the theoretically calculated and the practically achievable volume expansion ratio of a particular foamable resin. Thus, in order to compensate for the loss of the blowing gas that takes place during the foaming stage, the amount of CBA introduced into the polymer while preparing the foamable resin should be greater than the theoretically calculated. This paper establishes the relationship between the theoretically calculated level of CBA concentration in the foamable resin formulation and the actual experimentally-obtained volume expansion ratio in order to predict the necessary CBA concentration correction factor for a given volume expansion ratio and polymer material-blowing agent combination.
Producing polypropylene (PP) foams with satisfactory cell morphologies in rotational foam molding is feasible. However, the narrow interval between the melting temperature of PP and the onset decomposition temperature of the suitable chemical blowing agent (CBA), together with the low melt strength of PP at elevated temperatures, often represent the greatest obstacles in the foaming of PP. Experimental results revealed that the morphology of the foams obtained by processing PP pellets that have been pre-compounded with a CBA could be governed by either pellet sintering or cell coalescence. The viscosity of the basic PP resin and the processing temperature determine which of these two key factors will assume a predominating influence towards the foaming process. Desirable PP foam structures in compounding based rotational foam molding could be obtained only if pellet sintering takes place prior to the decomposition of the CBA and if the processing temperature during the foaming process is kept lower than the temperature of cell coalescence.
In a joint project with the German automotive industry, the Fraunhofer Institute, material suppliers, component-and mold manufacturers, a thermoplastic sandwich material has been developed. The goal is to offer a cost-effective material with increased mechanical properties to combine the advantages of In-Line-Compounded long fiber reinforced thermoplastics (LFT-ILC) or well-established thermoplastic semi-finished products like GMT and advanced thermoplastic TWINTEX® woven fabrics. These requirements are fulfilled by a sandwich which consists of outer layers of woven fabrics and a core layer of recycled material mainly of shredded TWINTEX®, GMT or LFT components or production waste. The foot support for the smart vehicle has been selected to evaluate the sandwich system.
A distinct difference was found between metallocene and Ziegler-Natta catalyzed linear low density ethylene copolymers (LLDPE) in the coextrusion with polypropylene (PP). A layer of amorphous material was hypothesized to form between PP/Ziegler-Natta LLDPE interface and not in the PP/metallocene LLDPE system. The presence of a weak amorphous interfacial layer was supported by the results of the T-peel test where the metallocene LLDPE system showed significantly higher level of adhesion between PP and LLDPE than the Ziegler-Natta LLDPE systems.
Blends of ethylene-styrene interpolymers (ESIs) are a model system for studying the miscibility of a-olefin copolymer blends. The phase behavior of partially miscible ESI blends, with styrene content difference 9-10 wt%, was characterized by phase diagrams. Blends were rapidly quenched from the melt to retain the phase morphology, and the volume fractions of the two phases were obtained from AFM phase images. Assuming monodisperse polymers, the phase composition was approximated by extrapolation of the relationship between blend composition and phase volume fraction. The blends exhibited an upper critical solution temperature (UCST). The UCST decreased with decreasing molecular weight and decreasing styrene content difference. Phase compositions were also obtained with an analysis that considered the molecular weight distribution. Calculated results indicated that phase compositions depended on the initial blend composition. The interaction parameter obtained with this approach was independent of molecular weight and was proportional to the square of the styrene content difference. The solubility parameter, extracted from the interaction parameter, agreed with literature values.
The morphology and mechanical properties of PP/LLDPE melt blends were compared and related to differences in adhesion. Blends of conventional Ziegler-Natta and metallocene PE's with conventional and high melt strength PP's were examined in terms of phase morphology by SEM and mechanical properties by tensile stress-strain measurements. By comparing blends of similar morphology, differences in tensile properties could be correlated to adhesion. Interfacial structures of yielded specimens observed with SEM were consistent with yield stress and modulus analyses that suggested differences in interfacial adhesion among the blend systems.
The injection molded part is for a very critical radar application and therefore its parabolic form has to be maintained through stringent application and environment conditions, while satisfying a number of other functional and quality requirements. A D-Optimal Design Of Experiments (DOE) analysis was run to identify an optimal process window in the four parameter design space, within which, ten very critical and tight-tolerance performance criteria were satisfied simultaneously. Prediction models generated based on the DOE analyses were shown to accurately represent the actual molding process. These models were then coded in a program to be utilized by molding engineers in process sensitivity analyses.
Recent technical breakthroughs in pad printing machinery; such as automatic ink viscosity control, automatic pad cleaning, and compact automation friendly designs will assure a place for modernized pad printing well into the future. Gone are the old-school open inkwell systems that had their share of problems in production. Many modern machines have also been designed with an approach that requires only one (or two) nesting fixtures, giving many molding facilities ultimate flexibility without massive tooling costs. With these new process control innovations having been developed, the pad printing process now allows easy integration into automated systems. To help further bring pad print systems into modern molding facilities, the ink systems utilized have also made many technical leaps forward.
The functionalization of polypropylene (PP) with maleic anhydride in the presence of supercritical carbon dioxide was studied. Supercritical carbon dioxide was used in this reactive extrusion system to reduce the viscosity of the polypropylene melt phase by forming a polymer-gas solution in order to promote better mixing of the reactants. For that purpose, a reactive extrusion system was developed to facilitate grafting of maleic anhydride onto PP under supercritical pressures in a section of a twin screw extruder. Subsequently, the effect of supercritical carbon dioxide on the level of grafting, product homogeneity and molecular weight was evaluated. Analysis of the products revealed that the use of supercritical carbon dioxide led to improved grafting when high levels of maleic anhydride were used. The experimental results showed no evidence of an improvement in the homogeneity of the product while melt index measurements showed a reduction in the degradation of polypropylene during the grafting reaction when low levels of maleic anhydride were employed.
We have developed a novel extruded foam structure that appears to have many advantages over current structures. The concept is a unique extension of the technology used to produce The Dow Chemical Company's STRANDFOAM* brand extruded polypropylene (PP) foam products. STRANDFOAM brand PP foam products are produced by extruding a gel made up of polymer and dissolved blowing agent through a die consisting of an array of holes. As the gel exits the die, it expands to form individual solid cylinders that coalesce into a solid plank. Hollow strand foam is produced in a similar fashion, except that the polymer gel is extruded through an array of annuli instead of an array of holes. This process produces hollow strands or tubes that expand and coalesce into a plank. The hollow strand structure gives the foam a host of interesting properties. For example, the hollow strand foam has excellent compressibility and recoverability compared to solid foam planks of similar composition. In addition, the new structure allows easier production of foam products with a low bulk density. Perhaps most intriguingly, it is possible to combine the hollow strand foam with solid strand or plank foam to produce products having two sets of performance attributes. There are numerous extensions of, and applications for, this technology. Examples include: impact energy absorbers; high impact packaging with built in drainage; low density hard to blow" foams like PET PC; leveling insulating board for concrete surfaces; combined thermal and acoustical insulation."
A food package is called upon to perform a number of functions, including • providing a hermetic seal, • protecting against the environment (e.g., controlled moisture or oxygen ingress or egress, puncture resistance, impact resistance), • safeguarding the flavor of the food, • allowing the product to be packaged on high speed filling lines, and • conveying information to the consumer (e.g., advertising and product contents). A single material often cannot provide these functions in an economical way. Hence, multilayer structures are prevalent in the packaging industry. A number of examples are illustrated. Several processes are used to make multilayer packaging films including extrusion coating, lamination, coextrusion and various combinations. This paper will deal primarily with coextrusion. Designing and manufacturing multilayer films present special challenges beyond those encountered when making single layer structures. In this paper we will review recent progress made in understanding four such challenges: interlayer adhesion, the effect the coextrusion process itself may have on properties, stiffness and curl.
This paper is aimed on the development of a constitutive model for NORYL GTX with the purpose of Finite Element (FE) prediction of the residual deformation, caused by thermo-mechanical cycling of structural elements. This material is a blend, produced by General Electric Plastics and widely used in automotive industry. Two brands of this thermoplastic: NORYL GTX 964 and NORYL GTX 974 are specially developed for automotive parts for in-line painting. The technology of in-line painting includes thermo-cycle with maximum temperature ~ 170°C. Therefore, the aim of this study is related to prediction of performance of thermoplastic parts during and after the thermo-mechanical cycles (of painting). For this purpose an earlier developed non-linear visco-elasticity model of relaxation type is employed. The model calibrated using the experimental data on sagging of single-side-clamped thermoplastic plates under thermal cycles. The ability of the model to predict independent events was validated basing on the data from the tests of double-sides-clamped plates, subjected to a thermo-mechanical loading. Difference between the model prediction and the results of controlling tests is less than 10% that confirms applicability of the model proposed.
Polymerization of ethylene using metallocene catalysts, particularly the constrained geometry catalysts (CGC), was studied. The main focus of the paper is on the control of chain microstructure in olefin polymerization using metallocene catalysts, particularly long chain branching in ethylene polymerization. Combined metallocene catalysts, consisting of CGC and a conventional metallocene catalyst, which only produces linear chains (linear catalyst), were used to manipulate long chain branching degree. The feasibility of this technique was verified using a mathematical model developed for the polymerization of ethylene in a semi-batch reactor using combined catalyst systems. Polymerization experiments were performed to verify the validity of the proposed technique and some of the modeling results. It was shown that by choosing a proper catalyst system and polymerization conditions chain microstructure could be tailor-made. Monte Carlo simulation was also used to study the structure and length of the branches in metallocene catalyzed ethylene polymerization. This information is essential for making any correlations between LCB degree and rheological properties.
Estimation of chemical composition distribution in ethylene/?-olefin copolymers using crystallization techniques is studied in this paper. Monte Carlo simulation was used to model the fractionation process in crystallization analysis fractionation (CRYSTAF). Five poly(ethylene/1-octene) samples synthesized with a single-site-type catalyst were used to verify the simulation results. It was proposed that the fractionation mechanism be controlled by the crystallization of the longest ethylene sequence in each chain. Good agreement between experimental and simulation results verified the validity of the proposed fractionation mechanism.
Thermal deformation is one of the major problems that affects a product's quality in plastic injection molding. It is very important to have an accurate evaluation of the thermal environment surrounding the injection mold, especially in the case of high precision or large product molding. This paper proposes an evaluation method of thermal environment with a consideration of both the effect of the resin solidification process and the product's geometric shape. With this method, the validity of the thermal environment in different cooling designs is discussed using numerical analysis.
Tensile strengths of linear vibration welds of nylons with different melting temperatures and glass contents were determined. Two different geometries were investigated: T-welds and a cylinder welded to a plaque. The study involved weld strengths for both homogeneous" welds where both components being welded were the same and for "heterogeneous" welds where the two components were made of nylons of different melt temperatures. It was determined based on the T-weld and the cylindrical-weld analysis that there is potential for dissimilar material weld combinations using vibration welding as the joining process. Relatively high weld strengths were obtained when PA 6 (and PA 66) was welded to the 3 different PPA's. For the short-glass-fiber materials one of the factors influencing weld strength was the difference in melt temperature between the two resins: greater difference in melt temperature resulting in lower weld strength. The long glass-fiber reinforced material which is predominantly PA 66 exhibited approximately the same weld strength regardless of the difference in melt temperature between it and the other nylon to which it was welded."
This paper presents an innovative design of a tandem extrusion system for fine-celled foaming of plastic/wood-fiber composites using a physical blowing agent (PBA). The plastic/wood-fiber composites utilize wood-fibers as reinforcing filler in the plastic matrix and are known to be advantageous over the neat plastics in terms of the materials cost and some improved mechanical properties such as stiffness and strength. However, these improvements are usually accompanied by sacrifices in the ductility and impact resistance. These shortcomings can be reduced by inducing fine-celled or microcellular foaming in these composites, thereby creating a new class of materials with unique properties. An innovative tandem extrusion system with continuous on-line moisture removal and PBA injection was successfully developed. The foamed composites, produced on the tandem extrusion system, were compared with those produced on a single extruder system, and demonstrated significant improvement in cell morphology, resulting from uniform mixing and effective moisture removal. The effects of both wood-fiber and PBA (CO2) content on the cell morphology and foam properties were studied. Increasing the CO2 content marginally improved the cell structure, whereas, increasing the wood-fiber content had an adverse affect. The effectiveness of a coupling agent was also evaluated. The cell morphology and foam properties showed improvement when the coupling agent was added.
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