Showing papers in "Polymer Engineering and Science in 1990"

A process for making microcellular thermoplastic parts

Abstract: A novel process to produce microcellular thermoplastic parts is described This is achieved by integrating the deformation process in the foaming cycle in such a way that the cell nucleation and growth processes are effectively uncoupled from deformation The nitrogen-polystyrene system is studied and the relationships between the essential process parameters are established It is experimentally shown that the pressures associated with deformation do not reduce the number of bubbles nucleated The process synthesized is demonstrated by making a microcellular polystyrene container

Chain structure, phase morphology, and toughness relationships in polymers and blends

Abstract: We present chain structure, phase morphology, and toughness relationships in thermoplastic polymers and polymer/rubber blends. In neat polymers, molecular aspects of craze/yield behavior are controlled by two chain parameters: entanglement density ν e and characteristic ratio C ∞ . The crazing stress is proportional to ν e 1/2 , and the yield stress is proportional to C ∞ . The dispersed rubber toughens a polymer/rubber blend mainly by promoting energy dissipation of the matrix. The toughening efficiency correlates with the rubber phase morphology and the chain structure of the matrix

Polymer Blends Containing Liquid Crystals: A Review

Abstract: This paper reviews the literature of polymer blends containing low and high molar mass liquid crystals. Low molar mass liquid crystals have been used as plasticizers for thermoplastic polymers and in applications such as electro-optics, optical recording media, and membranes. High molar mass liquid crystalline polymers have been primarily used in polymer blends as processing aids and as an incipient reinforcing phase for “self-reinforced” materials. This review discusses the phase behavior, rheology, and mechanical properties of these blends.

An experimental and anaiytical investigation of the large strain compressive and tensile response of glassy polymers

Abstract: In this investigation, the plastic flow of polycarbonate (PC) was examined by obtaining true stress-strain data over a range of strain rates at room temperature through homogeneous, uniaxial, constant strain rate compression testing to strains as high as 125 percent. Uniaxial compressive loading conditions give rise to a planar molecular orientation process which results in the observed strain hardening in compression. Uniaxial tensile tests on PC were also conducted. The necked region of the tensile specimen is being cold drawn resulting in a uniaxial state of orientation. Therefore, the observed macroscopic strain hardening in uniaxial tension distinctly differs from that obtained In uniaxial compression, giving different stress-strain curves. The major differences experimentally obtained between the large strain response in compression and tension indicate a need for an orientation-based model of the strain hardening process. The experimental program also acts to uncouple the effects of strain softening and strain rate providing more accurate data for future modeling of the true strain softening process. A constitutive law which directly relates the strain hardening response to the state of molecular network stretch in the polymer is used to model and analyze the experiments. The model is found to simulate the observed rate dependent yield and post yield strain softening and hardening of the compressive data over the entire range of strain rates very well. The model is then utilized in a finite element analysis of the tensile tests on PC. Numerical results compared favorably with the experimental data including: load vs, contraction curves, natural draw ratio, and the axial stress-strain response of the cold drawing region.

Morphology‐processing relationships in polyethylene‐polyamide blends

Abstract: Blends of polyethylene (PE) and polyamide (PA) were prepared by a melt mixing process. The dependence of the number average size An of the dispersed phase on hydrodynamic conditions not only of shear rate but also temperature, inter-facial tension, viscosity of the blends (WU's treatment), and dispersed phase concentration were studied. The analysis of PE-PA blend morphology shows An to be the result of a balance between coalescence and disruption of the particles in the melt, and to display a strong increase in particle size when the minor component concentration is enhanced. These observations can be explained by a change in the rheology of the system. These assumptions are confirmed by the insertion in the blend of an ethylenemaleic anhydride (EMA) copolymer that yields EMA-g-PA graft copolymer during mechanical processing. Formation of this graft copolymer has been indicated by a strong particle size reduction and its concentration was measured by infrared experiments. The EMA-g-PA copolymer seems to decrease the blend interfacial tension and to deter the coalescence process.

Composites from compounding wood fibers with recycled high density polyethylene

Abstract: The development of composites consisting of wood fibers and recycled plastics offers not only an opportunity to utilize an abundant natural resource but also a means to alleviate the serious plastics disposal problem. In this work, aspen fibers are incorporated into recycled high density polyethylene with a co-rotating inter-meshing twin-screw extruder to study processing-property relationships. Tensile, impact, and flexural strengths are measured as functions of fiber concentration. The effects of fiber pretreatment, screw configuration, and compounding temperature on the properties of composites are discussed.

Microspheres and microcapsules, a survey of manufacturing techniques Part II: Coacervation

Abstract: A methodological survey of coacervation/phase separation techniques employed for the preparation of microspheres and microcapsules is presented. Basic features of macromolecular coacervation are discussed, and a classification of different coacervation procedures (i.e., simple, complex, aqueous, and nonaqueous) is provided. Microsphere formation and microencapsulation techniques based on coacervation/phase separation of gelatin, gelatin-acacia, and ethylcellulose are described, and those of a wide range of other polysaccharide derivatives and synthetic polymers are tabulated. The dependence of microsphere/microcapsule characteristics on manufacturing parameters and performance evaluation of microspherical/microcapsular products are also discussed.

Observation of carbon black agglomerate dispersion in simple shear flows

Abstract: Experiments aimed at studying the mechanisms of agglomerate breakup due to the application of a simple shear flow field were performed in a cone and plate transparent device. Spherical compacts of carbon black (diameters 1-2 mm) in a range of different porosites were used in the experiments. Two distinct breakup mechanisms, denoted as “rupture” and “erosion”, were observed. The critical stress for erosion was found to be smaller than that for rupture. Once erosion starts, it continues for very long times. Rupture occurs shortly after reaching a critical stress and concludes abruptly. For this analysis of rupture, the dimensionless group a = (V.Y/K’@~), which is the ratio of applied stress to cohesive strength, was found to be a significant parameter for determining the final particle size distribution. The size analysis of fragments produced by shearing pellets for 1 minute showed a lognormal distribution function.

The interface in binary mixtures of polymers containing a corresponding block copolymer: Effects of industrial mixing processes and of coalescence

Abstract: In theories of the minor phase (domain) formation in polyblends rendered as emulsions it is usually assumed that the size and shape of the domains are the result of melt viscosity effects (Taylor, Wu) or viscoelasticity effects (VanOene, Elmendorp) being balanced by interfacial tension. This assumption would predict a monotonic decrease of the domain size to a final limiting size with increasing energy of mixing. However, a systematic study of the dependence of domain morphology on industrial mixing processes which was carried out on a “model” LDPE/PS (2/1) mixture and the related polyalloy (i.e., the same mixture with a corresponding block copolymer as compatibilizer) does not support this expectation. Doirain size was found to go through a minimum as mixing energy was increased. A similar minimum was seen in data on specific volume of the melt vs. mixing energy, which indicates a correlation between melt specific volume and domain size. Calculation of the approximate surface area of the domains using a simple model of domain shape indicated that total interfacial energy in the polyblend and/or polyalloy is a trivial part of the mixing energy introduced. These calculations also indicated that if compatibilizer was located entirely at the interface, the surface layer would have a thickness of about 90 nm. Some micrographs seem to show such a surface layer. We propose that an abrasion mechanism is responsible for the early stage of the dispersion process, and that the final domain size may be controlled by a dispersion-coalescence equilibrium. This is compared with the theories of final particle size proposed by VanOene and Wu.

Morphology and mechanical-properties of thermoplastic composites containing a thermotropic liquid-crystalline polymer

Abstract: Blends of two thermotropic liquid crystalline polymers (TLCPs), with brittle and ductile matrix materials were both injection molded and spun into fibers, in order to investigate the mechanism of in-situ mechanical reinforcement. In the injection molded samples, the TLCP was only moderately elongated into fibrils, and the mechanical properties were below predictions of the rule of mixtures. Fibers spun out of the blends contained numerous fine fibrils with nearly infinite aspect ratio, and as expected, the modulus increased linearly with the TLCP volume fraction, obeying the Tsai-Halpin equation for transversely isotropic composites. Wide angle X-ray diffraction measurements, as well as determination of the fiber-moduli, revealed that during spinning not only a macroscopic elongation of the fibrils was achieved, but also a considerable molecular orientation within the TLCP domains.

Influence of solvent and molecular weight on thickness and surface topography of spin‐coated polymer films

Abstract: The influence of polymer molecular weight, molecular weight distribution, and polymer-solvent interactions on the thickness and topography of spin-coated polymer films was examined. For films prepared from dilute solutions, highly volatile solvents or fair or “poor” solvents for the polymer adversely affect film surfaces causing nonuniformities (waves) to appear. However, if the concentration of these solutions is increased to approximately the concentration at which entanglements are formed, nearly uniform films are produced even if the solvent employed is highly volatile, such as dichloromethane. When toluene is employed as the solvent, which has a relatively low volatility and therefore forms nearly flat film surfaces, films prepared from dilute solution were found to have thicknesses, h, proportional to η Ω−0.49 for polystyrene and η Ω−0.49 for poly(methylmethacrylate) where ηo is the zero-shear rate solution viscosity and Ω is the rotational speed at which the films were prepared. These results suggest that the exponents associated with ηo and Ω may be nearly independent of the type of polymer used as long as flat films are produced. Finally, the molecular weight parameter most important in controlling final film thickness for films made from dilute solutions is Mv, the viscosity-average molecular weight.

Microspheres and microcapsules, a survey of manufacturing techniques: Part III: Solvent evaporation

Abstract: A methodological survey of microsphere formation and microencapsulation techniques based on solvent extraction/evaporation techniques is presented. Thus, basic features of solvent extraction and solvent evaporation processes, including droplet formation, droplet/particle stabilization, and solvent removal, are outlined. Preparation of a wide range of microspherical and microcapsular products based on biodegradable polyesters, polysaccharides, and nonbiodegradable polymers are discussed. Dependence of microcapsule characteristics on manufacturing parameters, as well as performance evaluation of microspherical and microcapsular products, are also briefly covered.

Polypropylene/polyethylene terephthalate blends compatibilized through functionalization

Abstract: In attempts to improve the compatibility of polypropylene with polyethylene terephthalate, an acrylic acid functionalized polypropylene was evaluated as the blend component in polyblends containing 40 percent by weight polyethylene terephthalate and compared with an unmodified polypropylene. The preliminary experiments in a batch laboratory mixer were followed by compounding in a co-rotating twin-screw extruder. Additives such as magnesium acetate and p-toluenesulfonic acid were evaluated as catalysts for potential interchange or esterification reactions that could occur in the melt. The blends were characterized through scanning electron microscopy, infrared spectroscopy, differential scanning calorimetry, and for mechanical properties. The results show that the functionalized polypropylene promotes a fine dispersed phase morphology, improves processability and mechanical properties, and modifies the crystallization behavior of the polyester component. These effects are attributed to enhanced phase interactions resulting in reduced interfacial tension (calculated as a 4-fold decrease). The presence of the additives does not, in general, improve any further the blend morphology and properties, or its processability.

Processing characteristics, structure development, and properties of uni and biaxially stretched poly(ethylene 2,6 naphthalate) (PEN) films

Abstract: Poly(ethylene 2,6, naphthalene dicarboxilate), PEN, is very similar to poly(ethylene terephthalate), PET, in its chemical structure and was, therefore, expected to exhibit similar processing characteristics. We, however, observed a few problems during stretching of PEN, the most important of which was necking behavior at 145°C, which is between Tg (117°C) and Tcc (195°C). This is usually observed in PET only when it is stretched close to or below Tg. At temperatures between Tg and Tcc (cold crystallization temperature) PET stretches rather uniformly. The temperature window for film stretching appears to be rather wide, but our results indicate that this is not the case. Films stretched to high stretch ratios become uniform due to propagation and final disappearance of necks as a result of stress hardening. Our attempts at stretching these films at higher temperatures indicated that necking is eliminated, but so is stress induced crystallization, which causes stress hardening (unless high stretching rates are employed). The presence of stress hardening is essential for obtaining high quality, uniform films of these polymers. In addition, at high temperatures thermally activated crystallization which starts dominating the structure development, detrimentally affects the general appearance of the films. In brief, the PEN films we investigated have a narrower processing window than was anticipated based on their thermal behavior alone. At elevated temperatures the films are sensitive to the rate of stretching even more than typical PET processed at comparable conditions. The uniformity of the films depends on the stretch ratio, stretching mode, ratio(s) and rates and temperature. WAXS studies on the films indicate that the macromolecules packed into the low temperature crystal modification. In addition, WAXS pole figure studies suggest that naphthalene planes preferentially orient parallel to the film surface during biaxial stretching. The biaxially stretched films were observed to exhibit a bimodal chain orientation as evidenced by pole figure analysis of the (010) planes.

A numerical study of bubble growth during low pressure structural foam molding process

Abstract: The bubble size distribution created by the expanding foam plays a key role in controlling the load-bearing and other mechanical properties of the manufactured structural foam part. A numerical method to study the bubble growth and predict the bubble size distribution in polymeric foams is presented. On the microscopic scale, a cell model has been used. A cell is a system composed of a hypothetical spherical gas bubble and an envelope of polymer with constant mass surrounding the bubble. On the macroscopic scale, the foam has been modeled as a compressible medium consisting of a number of cells growing in close proximity to each other. The coupled system of the bubble growth equations for a cell and the field equations for the polymeric fluid are solved numerically to predict the spatial bubble size distribution and the flow front movement during the expansion process. The influence of different dimensionless parameters on the growth of spatially distributed bubbles and on the relative reduction in the transient bulk foam density, under isothermal condition; has been predicted. The existence of an axial pressure gradient in the mold due to the spatial variation of bubble growth is demonstrated through numerical experiments.

A barometer of crystallization rates of polymeric materials

Abstract: A crystallization rate coefficient (CRC) parameter is introduced which has allowed a direct comparison of the crystallization rates of various polymers on a single scale for the first time. Basically, CRC represents a change in cooling rate required to bring about 1°C change in the supercooling of the polymer melt. For the polymers studied, this value varies between 35 h−1 (polyethylene terephthalate) and 155 h−1 (poly(tetrafluoroethylene)) and jumps to 295 h−1 for indium metal; the precision being better than ±5 percent. The reliability of CRC has been tested against the established trends e.g., (i) a large decrease in the crystallization rate of polyethylene terephthalate with increase in molecular weight, (ii) a lower crystallization rate of the “virgin” nylon 6 as compared to the processed nylon 6 resin, (iii) an increase in the crystallization rate of nylon 6 in the presence of nucleating agents, and (iv) a dramatic increase in crystallization rate as we go from poly(ethylene terephthalate) to poly(tetrafluoroethylene) and then from poly(tetrafluoroethylene) to the metals. The significance of the CRC barometer is discussed in the light of prior difficulties in directly comparing the crystallization rates of polymers.

Methanol-Induced crack healing in poly(methyl methacrylate)†

Abstract: Crack healing in poly(methyl methacrylate) (PMMA) by methanol treatment at 40°C–60°C has been investigated. It is found that the methanol treatment reduces the glass transition temperature in PMMA. Crack healing only occurs at an operating temperature higher than the effective glass transition temperature. There are two distinctive stages for crack healing based on the recovery of mechanical strength. The first stage corresponds to the progressive healing due to wetting, which has a constant crack closure rate at a given temperature. Immediately following the first stage, the second stage corresponding to diffusion enhances the quality of healing behavior. The surface morphologies obtained during healing and after fracture tests confirm these two stages. By comparing the fracture stress with the fractography, the fracture surface for stage I of crack healing is coplanar to the original crack surface. On the other hand, the original crack surface is destroyed in stage II of crack healing. It occurs in the region incorporating the original healed surface and appears to be like the Virgin fracture surface. It is also found that the tensile fracture stress of PMMA treated by methanol can recover to that of the virgin material. In addition, it is interesting to find that after sustained methanol treatment, the “snake bone” phenomenon on the fracture surface emerges.

The plane-strain essential work of fracture as a measure of the fracture toughness of ductile polymers

Abstract: We have extended the essential work of fracture technique to allow for the determination of the plane-strain essential work of fracture. The new technique is to measure the specific work of fracture as a function of ligament length in deeply double edge notched samples. This type of data is then experimentally corrected to remove the plastic work of fracture and leave only the essential work of fracture as a function of ligament length. By extrapolating the essential work of fracture to zero-ligament length, we claim to be measuring the plane-strain essential work of fracture. This new technique was applied to two rubber toughened nylons and to a series of polyethylenes. The plane-strain essential work of fracture was found to be independent of thickness. Where comparison can be made to J-integral testing, the plane-strain essential work of fracture was similar to the critical J-integral, JIc.

Numerical simulation of the cast film process

Abstract: The cast film process is studied and a numerical simulation is performed using a finite element method. Two dimensional equations involving the thickness and a mean velocity are used by considering the film as a thin layer. The process is assumed to be isothermal between the die exit and the quick cooling on the chill roll. The width of the film is computed iteratively as a free surface problem by means of a finite element method. The thickness distribution of the film is obtained by a finite volume method. This approach allows modeling of the well known “dog-bone” or “edge bead” defect with a reasonable computation time.

Viscoelastic material characterization and modeling for polyethylene

Abstract: An extensive set of stress relaxation and constant strain rate tests for characterizing the mechanical responses of a medium density polyethylene and a high density polyethylene that are commonly used in natural gas distribution piping is described and analyzed. The development of coherent master curves for the relaxation modulus, maximum stress, and the time-to-failure for pressurized pipes through a combination of both horizontal and vertical shifting is presented. The relaxation data are used to develop a nonlinear Viscoelastic material model. The model is assessed by making comparisons of the predicted stress-strain response with the measured response in the constant strain rate tests.

Characterization and processing of blends of polyethylene terephthalate with several liquid crystalline polymers

Abstract: Blends of an engineering thermoplastic, poly(ethylene terephthalate) (PET), and two liquid crystalline polymers (LCPs) viz., copolyesters of PET and parahydrox-ybenzoic acid (PHB) in 40/60 mole percent (LCP60) and in 20/80 mole percent (LCP80) were prepared. A blend of LCP60 and LCP80 in 50/50 weight percent (LCP60-80) was blended with PET. Both flat films and rods were extruded and their properties examined. The morphology of the films investigated using Scanning Electron Microscopy (SEM) revealed that the LCP phase remained as dispersed droplets in the PET matrix. In spite of the lack of fibrillation in these films, the mechanical properties were enhanced to some extent with a maximum at 10 weight percent of the LCP phase. However, in the case of the rods thin fibrils of the LCP phase of the order of 1 μm in diameter were observed provided the composition of the LCP was 20 weight percent or greater. This success In achieving fibrillation is through to be due to the extensional flow fields present at the entrance of the capillary die and the fact that a short L/D ratio die was used. Differential Scanning Calorimetry (DSC) thermograms of the extruded films indicated that the LCP phase may act as a nucleating agent for the crystallization of PET. Rheology of the blends revealed that the complex viscosity of the blends is not much different from that of pure PET. This is attributed to the partial miscibility of the two components. Based on the DSC results and residence times in the extruder, it is concluded that no significant transesterification reactions appear to have: taken place in the blends. The rheology is studied further with respect to the cooling behavior of the pure components and factors important to the fibrillation of the LCP phase and the formation of in-situ reinforced composites are discussed.

Electrical and dielectric properties of segregated carbon black–polyethylene systems

Abstract: The electrical and dielectric properties of compression-molded segregated polyethylene-carbon black mixtures are described in the frequency range between 10 and 8000 kHz as a function of frequency, temperature, and carbon black loading. The segregated systems investigated exhibit insulator-conductor transitions in the range 0.25–0.65% (volume/volume) carbon black. The dielectric constant and the dissipation factor of the conductive samples are relatively very high in the frequency range studied. The dielectric constant increased sharply with the carbon black concentration, and then increased moderately beyond the insulator-conductor transition. The dissipation factor-concentration curves for different carbon blacks show maximum values in the vicinity of the critical concentration values. The dielectric properties of these systems are discussed in terms of interfacial Maxwell-Wagner polarization effects.

Energy absorption characteristics of polymeric foams used as cushioning materials

Abstract: The “Efficiency of Energy Absorption” (or “Efficiency”) and “Ideality” parameters were evaluated for several plastic foams and were found to be very useful in choosing an appropriate cushion for the protection of a specific product. The maxima in these parameters were found to be in the same range of stresses, when derived from dynamic measurements or from predicted stress-strain curves based on previously proposed “Reference” and “Modified Boltzman Superposition” Models. For the rate independent foams the maxima in these parameters do not change with strain rate and can therefore be derived from slow, constant rate (“static”) experiments. For rate dependent foams however, the maxima from “static” measurements were found to be in a lower range of stresses than those derived from dynamic ones. As a result, slow rate compressive measurements do not predict well the behavior of the foams during impact and the use of the “Reference” and “Modified Boltzman Superposition” Models is required for good predictions. It was found that the suffer PS and PE foams attain maximum Efficiency and Ideality at higher stresses than the more flexible PUR foams.

Numerical prediction of three-dimensional fiber orientation in Hele-Shaw flows

Abstract: A numerical technique is developed to determine the three-dimensional fiber orientation in complex flows. The fiber orientation state is specified in terms of orientation tensors, which are used in several constitutive models. This method is applied to quasi-steady state Hele-Shaw flows in order to predict the flow-induced fiber orientation during injection molding at zero volume fraction limit. At the inlet, a number of fibers are introduced at a specified rate into the flow and each fiber location is traced during the mold filling. Along these determined paths, the independent components of fourth order orientation tensors are solved, describing the orientation state. The numerical grid generation technique, which is suitable for complex mold shapes, is employed for the flow solution. Orientation ellipsoids are calculated from the second order tensors and are used to present the fiber orientation results. The numerical solutions are obtained for channel and converging flows. Planar, longitudinal, and transverse orientation results are generated from the orthogonal projections of the orientation ellipsoids.

Yield behavior of polypropylene filled with CaC03 and Mg(OH)2. I: “Zero” interfacial adhesion

Abstract: The composition dependence of tensile yield stress (σyc), and the effects of filler particle shape and surface treatment were investigated for polypropylene (PP) filled with CaCO3 or with Mg(OH)2. Poor adhesion between PP and CaCO3 accounted for a decrease of σyc with increasing σf. In spite of the poor adhesion between PP and Mg(OH)2, σyc either slowly decreased with increasing Vf or remained constant up to Vf = 0.25. Surface treatment of the fillers facilitated better dispersion in PP. On the other hand, σyc was reduced due to the lower thermodynamic work of adhesion (WA). The semiempirical one-parameter equation proposed by Turczanyi, Pukanszky, and Tudus (TPT) was considered plausible and was employed in the study of the effects of matrix-filler interaction.

Optimization of injection molding design. Part I: Gate location optimization

Abstract: The placement of a gate in an injection mold is one of the most important variables of the total mold design. The quality of the molded part is greatly affected by the gate location, because it influences the manner in which the plastic flows into the mold cavity. Some defects, such as weldline and overpack, can be effectively controlled only by the gate location. Therefore, the product quality can be greatly improved by determining the optimum gate location. In this paper, we develop a general methodology for gate location optimization. We first quantify quality in terms of flow simulation outputs. We can thus assess detrimental effects such as warpage and dimensional instability as a function of the independent variable, which is in this case the gate location. Next we develop methods to search for the optimum gate location. The search method introduced in this paper is a method that combines a deterministic hill climbing search with a stochastic annealing search method. The method is appropriately called simulated annealing and hill climbing (SANHIL). The criteria used for evaluation during the search process are a function of the flow simulation outputs. We demonstrate the success of the method for a complex industrial mold. The approach is applicable to any complex mold geometry and any plastic.

The morphology of extruded blends containing a thermotropic liquid crystalline polymer

Abstract: Three different polymer blends consisting of an isotropic matrix and a ther-motropic liquid crystalline polymer (LCP) as the reinforcement were extruded. Polycarbonate (PC) and polyetherimide (PEI) were the two matrices, and the LCPs used were Vectraw A950 of Hoechst-Celanese, a copolyester of hydroxybenzoic and 2,6 hydroxynaphthoic acids and an LCP of Granmont Inc., a condensation polymer of terephthalic acid, (l-phenylethyl)hydroquinone, and phenylhydroqui-none. These extrudates were characterized by a quantitative morphological technique to determine the percentage of LCP present as fibrils and the average domain diameter. These experimental observations were then coupled with the component Theological behavior and a simple heat transfer analysis to explain the morphology and property differences between the blends. Blends with the Granmont LCP showed no appreciable increase In the quantity of fibrils with draw ratio, for example, whereas the amount of fibrils in Vectra® blends tended to increase to a plateau with draw. The tensile modulus of the blends agreed well with composite theory, with average fibril moduli of 24.6 GPa and 23,3 GPa for Vectra® and the Granmont LCP, respectively. These differences can be explained in terms of the cooling behavior of the LCPs.

Measurement of thermal conductivity of polymer melts by the line‐source method

Abstract: An apparatus has been developed for the measurement of thermal conductivity of polymer melts. Based on the transient “line source method,” it is ideally suited to these materials because measurements can be made quickly, before the onset of thermal degradation. Also, little or no sample preparation is required. A number of commercial polymers have been tested, including some glass-fiber filled composites.

Thermal contact resistance in injection molding

Abstract: This paper describes a method for obtaining the thermal contact resistance (TCR) between an injection molded part and its mold. Due to the absence of TCR the simulated cooling times obtained from Polycool II, a computer aided engineering (CAE) package for cooling simulation of injection molding, have compared poorly with both field and experimental data. This paper shows that an improvement in the accuracy of the simulated data results from making TCR an Input to Polycool II. TCR was obtained through a combination of experimental and analytical procedures. Experimental work was performed to obtain the part surface temperature distribution and the inside cavity pressure gradient. The part surface temperature distribution was then used as a boundary condition in the thermal analysis. The inside cavity pressure gradient was utilized as a basis for determining the inside cavity shrinkage. The results show that due to the thermal expansion of thermoplastics, the compressibility of the plastic melt, and the mold deformation, the inside cavity shrinkage is reduced as the thickness of the part is increased. Therefore, the TCR value of a thicker part is lower than that of a thinner part. The effects of both part thickness and process parameters, such as temperature and pressure, on TCR are also discussed.

Rheology-morphology relationships in nylon 6/liquid-crystalline polymer blends

Abstract: Extrusion measurements have been carried out on blends of nylon 6 and a liquid-crystalline copolyesteramide (LCP). The flow curves at low temperature show a behavior similar to that of pure LCP with a rapid rise of the viscosity at low shear rates. At high shear rates the viscosity is lower than that for each of the two components. This minimum has been attributed to the lack of interactions between the two phases and to the formation of fibrils of the LCP phase. The SEM analysis shows, indeed, that fibrils of the LCP phase are produced in the convergent flow at the inlet of the capillary at high shear rates. These fibrils are lost during the flow in the long capillary.