Showing papers in "Polymer Engineering and Science in 2004"

Strain hardening in polypropylenes and its role in extrusion foaming

Abstract: Extensional viscosity of several polypropylene polymers and their blends was measured and the foam processing of these blends using carbon dioxide blowing agent was studied. Foaming was carried out on a co-rotating twin-screw extrusion line, with a gear pump to build pressure. A linear isotactic polypropylene and two branched polypropylenes were considered. The uniaxial extensional viscosity was quantified and the foam characterized based on bulk density, cell size, and cell concentration. The linear polymer exhibits no strain hardening, while both branched polymers show pronounced strain hardening. Blends of low concentrations of branched polymer in the linear polypropylene show significant strain hardening down to 10-wt% branched polypropylene. Strain hardening is expected to prevent cell coalescence and lead to higher cell concentrations. The branched polymers were found to have a lower cell concentration than the linear polymer. Yet blends of linear and branched polypropylenes attained a cell concentration higher than either of the neat polymers. This suggests that even small amounts of branched polypropylene blended in linear polypropylene can improve the foaming process. Polym. Eng. Sci. 44:2090–2100, 2004. © 2004 Society of Plastics Engineers.

Thermoset rubber/layered silicate nanocomposites. Status and future trends

Abstract: This paper surveys the recent achievements with thermoset rubber/layered silicate nanocomposites considering their production methods, cure characteristics, structure and basic properties (mechanical, thermal and barrier). It was shown that the properties of the “nanoreinforced” rubbers strongly depend on the dispersion state of the silicate. The latter can be influenced by various methods related to the production route (latex, solution or melt compounding), silicate type (natural and artificial origin mostly affecting the aspect ratio), surface modification of the silicate (chemical buildup of the surfactant used for “organophilization”), recipe (curatives, activators, compatibilizers, etc.) and compounding parameters (temperature, time, shear rate, etc.). A peculiar skeleton-type reinforcing structure can be produced by latex compounding. Solution and melt intercalation techniques usually result in nanocomposites containing silicate layers in both intercalated and exfoliated forms. To detect the related structure the combined use of transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques is inevitable. Confinement and deintercalation of the organophilic layered silicates occurring during compounding can be circumvented by the proper selection of the surfactants. Based on the present praxis some tendencies for future R&D activities with rubber nanocomposites were deduced and a strong impetus was forecasted to them owing to the fast development with thermoplastic/layered silicate systems. Polym. Eng. Sci. 44:1083–1093, 2004. © 2004 Society of Plastics Engineers.

Crystallization kinetics of poly(L‐lactide) in contact with pressurized CO2

Abstract: The effect of CO 2 on the isothermal crystallization kinetics of poly(L-lactide). PLLA, was investigated using a high-pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO 2 . It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO 2 dissolved in the PLLA. The crystallization rate was accelerated by CO 2 at the temperature in the crystal-growth rate controlled region (self-diffusion controlled region), and depressed in the nucleation-controlled region. CO 2 has also decreased the glass transition temperature. T g , and the melting temperature. T m . As a result, the CO 2 -induced change in the crystallization rate can be predicted from the magnitudes of depression of both T g and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO 2 were also investigated using a wide angle X-ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO 2 , although the presence of CO 2 did not change the crystalline structure.

The effect of long chain branching on the processability of polypropylene in thermoforming

Abstract: Linear polypropylene was modified by reaction with peroxydicarbonates in a twin screw extruder to obtain varied degrees of long chain branching. The melt strength and the elasticity of the modified polymers were found to increase with the modification. The processability in foaming and thermoforming processes improved with branching and showed an optimum, beyond which higher degrees of long chain branching appeared not to help any further. The branched PP samples showed distinct strain hardening in the elongational viscosity, which was absent from the original linear melts. Melt strength, elasticity and strain hardening increased with the increase of the number of long chain branches on the main chain. The effect of molecular weight and molecular weight distribution of the precursor on the improvement of the processability of the polymer was examined. Polym. Eng. Sci. 44:973–982, 2004. © 2004 Society of Plastics Engineers.

Optimal process conditions of shrinkage and warpage of thin-wall parts

Abstract: Optimal process conditions of thin-wall injection molding of a cellular phone cover were investigated with the consideration of interaction effects between process parameters. L27 experimental tests based on Taguchi's method were performed, and then Cyclone Scanner, PolyCAD and PolyWorks were used to measure the shrinkage and warpage of the thin-wall injected parts to determine the optimal process conditions. Based on the results of the analysis of variables and the F-test, interaction effects for each observed factor were determined. The results indicated that the packing pressure was the most important process parameter affecting the shrinkage and warpage of the thin-wall part. The optimal process conditions were different for the shrinkage and the warpage. This was because during the injection process, the mechanisms affecting shrinkage or warpage were different. Compared with the results obtained with simplified thin-wall parts in the literature, it was found that the geometry of a real commercial part did affect the optimal process conditions and the order of influence of process parameters. The optimal process conditions determined by Taguchi's method for reducing the shrinkage and warpage were verified experimentally in this work. Polym. Eng. Sci. 44:917–928, 2004. © 2004 Society of Plastics Engineers.

Measurement and prediction of diffusion coefficients of supercritical CO2 in molten polymers

Abstract: The solubility and diffusivity of supercritical carbon dioxide (sc-CO2) in low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), ethylene-ethylacrylate copolymer (EEA) and polystyrene (PS) were measured at temperatures from 150°C to 200°C and pressures up to 12 MPa by using the Magnetic Suspension Balance (MSB), a gravimetric technique for gas sorption measurements. The solubility of CO2 in each polymer was expressed by Henry's constant. The interaction parameter between CO2 and polymer could be obtained from the solubility data, and it was used to estimate the Pressure-Volume-Temperature relationship and the specific free volume of polymer/CO2 mixtures. The diffusion coefficients were also measured by the MSB for each polymer. The resulting diffusion coefficients were correlated with the estimated free volume of polymer/CO2 mixture. Combining Fujita's and Maeda and Paul's diffusion models, a model was newly developed in order to predict diffusion coefficients for the polymers studied. Polym. Eng. Sci. 44:1915–1924, 2004. © 2004 Society of Plastics Engineers.

Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application

Abstract: This study aimed at developing highly conductive, lightweight, and low-cost bipolar plates for use in proton exchange membrane fuel cells. Injection and compression molding of carbon-filled polypropylene, PP, and polyphenylene sulfide, PPS, were used to fabricate the bipolar plates. Loadings up to 60 wt% in the form of graphite, conductive carbon black, and carbon fibers were investigated. The developed compositions have a combination of properties and processability suitable for fuel cell bipolar plate manufacturing, such as good chemical resistance, sufficient fluidity, and good electrical and thermal conductivity. Two bipolar plate designs were successfully fabricated by molding the gas flow channels over aluminum plates to form a metallic/polymer composite plate or simply by direct injection molding of the conductive polymer composite. For the first design, overall plate volume resistivities of 0.2 and 0.1 Ohm-cm were respectively attained using PP and PPS based blends as the conductive overmolded layer. A lower volume resistivity of around 0.06 Ohm-cm was attained for the second design with injection molded plates made of the PPS-based blend. Polym. Eng. Sci. 44:1755–1765, 2004. © 2004 Society of Plastics Engineers.

Cyclic deformation behavior of an epoxy polymer. Part I: Experimental investigation

Abstract: A series of uniaxial cyclic tests were carried out on solid cylindrical specimens of an epoxy resin, Epon 826/Epi-Cure Curing Agent 9551. The focus of the study was to investigate time-dependent viscoelastic behavior of this thermosetting polymer material under cyclic loading and to develop a constitutive model with the capabilities to simulate the observed deformation response. The tests include stress-controlled or strain-controlled cyclic loading with/without mean stress or mean strain at various amplitudes and loading rates. It was found that the cyclic stress-strain response of this material is amplitude-dependent and rate-dependent, and the response to axial tension is different from that in compression. The stress-strain loops exhibit more pronounced nonlinearity with high amplitudes or low loading rates. For stress-controlled cyclic loading with mean stress, ratcheting strain is accumulated, which is of viscoelastic nature, and this is confirmed by its full recovery after load removal. For strain-controlled cyclic loading with mean strain, the mean stress relaxation occurs, which contributes to the observed longer life in comparison to the stress-controlled cyclic loading with mean stress. Polym. Eng. Sci. 44:2240–2246, 2004. © 2004 Society of Plastics Engineers.

Relationship between cell morphology and impact strength of microcellular foamed high‐density polyethylene/polypropylene blends

Abstract: Polymer blends, such as those resulting from recycling postconsumer plastics, often have poor mechanical properties. Microcellular foams have been shown to have the potential to improve properties, and permit higher-value uses of mixed polymer streams. In this study, the effects of microcellular batch processing conditions (foaming time and temperature) and HDPE/PP blend compositions on the cell morphology (the average cell size and cell-population density) and impact strength were studied. Optical microscopy was used to investigate the miscibility and crystalline morphology of the HDPE/PP blends. Pure HDPE and PP did not foam well at any processing conditions. Blending facilitated the formation of microcellular structures in polyolefins because of the poorly bonded interfaces of immiscible HDPE/PP blends, which favored cell nucleation. The experimental results indicated that well-developed microcellular structures are produced in HDPE/PP blends at ratios of 50:50 and 30:70. The cell morphology had a strong relationship with the impact strength of foamed samples. Improvement in impact strength was associated with well-developed microcellular morphology. Polym. Eng. Sci. 44:1551–1560, 2004. © 2004 Society of Plastics Engineers.

Polypropylene/graphite nanocomposites by thermo-kinetic mixing

Abstract: Polypropylene (PP)/graphite nanocomposites have been prepared by melt-mixing PP with different levels of graphite (G) and graphite oxide (GO) using maleated PP(PP-g-MA) and graphite oxide (GO) as interface modifiers. Melt-mixing was achieved using a Gelimat, a high-speed thermo-kinetic mixer. The Gelimat system is specifically designed to handle difficult compounding and dispersion applications by completely mixing, heating and compounding products within a few minutes. Therefore, the thermal history of the compounded polymer is very short, which limits degradation. Interfacial modification by addition of maleated PP and graphite oxide is essential for producing PP/G nanocomposite. The graphite oxide then interacts with the maleic anhydride group of the PP-g-MA. The structure and properties of PP/PP-g-MA/GO/G nanocomposites were compared by different techniques. Evidence of the nanoscale dispersion of graphite sheets within the PP were provided by wide-angle X-ray diffraction (WAXD) and supported by scanning electron microscopy (SEM). The high mechanical shear stresses generated by the Gelimat greatly reduced the ordering initially measured by WAXD between graphite sheets and sheet aggregates, indicating a dispersion of the graphite in the polymer to the extent that graphite particles could hardly be observed by SEM. It was found that the addition of PP-g-MA and GO leads to excellent dispersion of G within the PP matrix. The flow behavior of the material was also studied by means of a parallel-plate rheometer. The addition of graphite to PP caused a drastic change in the flow behavior of PP. The thermal degradation behavior, studied using thermogravimetric analysis (TGA), showed higher thermal stability of the nanocomposite than that of pure polypropylene. The dispersion of the graphite in the resin promoted the nucleation of β crystallites in PP. The β crystallites, normally less abundant than α crystallites in pure PP, were found to constitute the dominant phase in the nanocomposite.

Effects of carbon nanotubes on the crystallization behavior of polypropylene

Abstract: The effect of the incorporation of single-walled carbon nanotubes (SWNTs) on the PP crystallization kinetics is investigated by thermal analysis, microscopy and Raman spectroscopy. The results of the investigation show that SWNT acts as a strong nucleation agent. A marked decrease of the half-time of PP crystallization as well as a sensible increase of the overall crystallization rate is observed in the presence of SWNTs. Moreover, these effects are already noticeable at the lowest SWNT content in the composite with slow further effects at higher concentrations, suggesting a saturation of the nucleating action at the higher concentrations studied. The Avrami model can represent the crystallization kinetics of PP in the composite. The kinetic curves obtained under non-isothermal conditions confirm the results obtained in isothermal tests and demonstrate the nucleation ability of SWNTs on the PP crystallization. Raman spectroscopy and scanning electron microscopy (SEM) are successfully applied to demonstrate that in the composite films, the changes in the crystallization kinetics can be explained in terms of the changes of the distance between nanotubes in bundles following a different intercalation of the polymer matrix.

Polyolefin nanocomposites: Formulation and development

Abstract: The preparation of nanoclay-reinforced polypropylene nanocom posites by means of melt processing was investigated. In order to optimize the dispersion of the nanoclay and the nanoclay-matrix interface, experiments were performed with three different nanoclays, two different maleic-anhydride–grafted PP coupling agents, and two different mixing procedures. The physicochemical and mechanical properties of the prepared samples were characterized by means of various techniques. The coupling agents increase the degree of clay intercalation and exfoliation, the latter resulting in part from a “peeling off” mechanism. Significant improvements in tensile and flexural strength and modulus are obtained with Cloisite® 15A nanoclay and a coupling agent characterized by high molecular weight and low grafting content, and these improvements are also accompanied by an increase in Izod impact strength. Little difference was observed between the two mixing procedures used. The improvements were not as pronounced when the coupling agent was characterized by low molecular weight and high grafting content, or when Cloisite® 30B nanoclay was used. In the latter case, there was evidence of greater thermal instability than for Cloisite® 15A. Polym. Eng. Sci. 44:1212–1219, 2004. © 2004 Society of Plastics Engineers.

A phenomenological constitutive model for foams under large deformations

Abstract: A nonlinear phenomenological constitutive model applicable to structural foams subjected to large deformations is proposed. The five-parameter model can fully capture the three fundamental features of stress-strain response, i.e., linearity, plasticity-like stress plateau, and densification phases, when subjected to compressive loads. Moreover, the parameters of the model can be systematically varied to capture the influence of initial density of foams that may be responsible for changes in yield stress and hardening-like or softening-like behavior under various confinement conditions. The model was successfully applied to capture the stress-strain response of two structural foams of different initial densities when subjected to uniaxial compression without lateral confinement and unixial compression under rigid confinement. Polym. Eng. Sci. 44:463–473, 2004. © 2004 Society of Plastics Engineers.

Recycled poly(ethylene terephthalate) chain extension by a reactive extrusion process

Abstract: A commercial-scale reactive extrusion processing system for recycled poly(ethylene terephthalate) (PET) flakes with an added chain extender, pyromellitic dianhydride (PMDA), was investigated. The PMDA concentration was varied with the intention of reaching a higher recycled PET intrinsic viscosity ([η]). The effect of changing the extruder residence time on the system's stability and the recycled PET [η] was also investigated. Reactive extruded PET with a PMDA concentration up to 0.3 wt% was found to have a higher [η] and lower carboxyl content than recycled PET processed in a normal extrusion system. A shift in [η] of about 0.18 dl/g was obtained with a 0.3 wt% PMDA concentration. A PMDA concentration above 0.3 wt% produced chemical, thermal and hydrodynamic instability in the system, causing crosslinking reactions and gel formation. The reactive extrusion system was stable at low residence time (45 s) and moderate (0.15 wt%) PMDA concentration; however, using 0.2 wt% PMDA produced higher reactive extruded recycled PET [η] with lower carboxyl content than other PMDA concentration levels examined. Residence times higher than 45 s produced higher reactive extruded recycled PET [η]. Reactive extruded recycled PET was also tested for mechanical properties. Polym. Eng. Sci. 44:1579–1587, 2004. © 2004 Society of Plastics Engineers.

Study of injection molded microcellular polyamide-6 nanocomposites

Abstract: This study aims to explore the processing benefits and property improvements of combining nanocomposites with microcellular injection molding. The microcellular nanocomposite processing was performed on an injection-molding machine equipped with a commercially available supercritical fluid (SCF) system. The molded samples produced based on the Design of Experiments (DOE) matrices were subjected to tensile testing, impact testing, Dynamic Mechanical Analysis (DMA), and Scanning Electron Microscope (SEM) analyses. Molding conditions and nano-clays have been found to have profound effects on the cell structures and mechanical properties of polyamide-6 (PA-6) base resin and nanocomposite samples. The results show that microcellular nanocomposite samples exhibit smaller cell size and uniform cell distribution as well as higher tensile strength compared to the corresponding base PA-6 microcellular samples. Among the molding parameters studied, shot size has the most significant effect on cell size, cell density, and tensile strength. Fractographic study reveals evidence of different modes of failure and different regions of fractured structure depending on the molding conditions. Polym. Eng. Sci. 44:673–686, 2004. © 2004 Society of Plastics Engineers.

Melt processing of PA‐66/clay, HDPE/clay and HDPE/PA‐66/clay nanocomposites

Abstract: Polyamide 66/clay, high-density polyethylene (HDPE)/clay and HDPE/PA66/clay nanocomposites were prepared, using a twin-screw extruder. The nanocomposites were characterized by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), optical microscopy and tensile testing. Effects of processing conditions and clay modifier were evaluated. The results show that exfoliation in the twin-screw extruder is enhanced by the incorporation of mixing and shearing elements and high residence times. Compatibility of the clay modifier with the polymer matrix plays an important role in exfoliation. Clay does not influence the crystal form, melting temperature or crystallinity of PA-66 and HDPE. However, it acts as a nucleation agent, increases marginally the crystallization temperatures, and reduces the crystallite size. Clay in the blend nanocomposites acts as a compatibilizer and changes the morphology of the blend. TEM micrographs suggest the presence of an exfoliated structure in PA-66 and an intercalated structure in HDPE.

Mechanical, thermal, and barrier properties of NBR/organosilicate nanocomposites

Abstract: Nanocomposites of intercalated and exfoliated organosilicates in acrylonitrile butadiene rubber (NBR) were prepared by a solution-blending method. The dispersion and intergallery spacings of organosilicates in these nanocomposites were examined by transmission electron microscopy and X-ray diffraction. Dramatic enhancements in the mechanical and thermal properties of NBR are found by incorporating less than ten parts of organosilicate. In particular, the addition of ten parts of organosilicate provided a more than sixfold increase in tensile strength, a twofold increase in M500, and 168% and 39% enhancements in tear strength and elongation at break compared with pure NBR. The degradation temperature for NBR with ten-parts' loading of organosilicate was 25°C higher than that of pure NBR. In addition, the relative vapor permeability of the NBR nanocomposites for water and methanol were 85% and 42% lower, respectively, than that of pure NBR. Polym. Eng. Sci. 44:2117–2124, 2004. © 2004 Society of Plastics Engineers.

Spectroscopic, transport, and morphological studies of polyaniline doped with inorganic acids

Abstract: A conducting polymer, polyaniline, was synthesized by chemical polymerization using different inorganic acids, such as HCl, H2SO4, HClO4, HNO3, and H3PO4, as protonic acid media. The synthesized polymers were characterized using UV-visible and FT-IR spectroscopy. A granular type of morphology was observed under SEM for HCl, HNO3, and H3PO4 doped polyanilines. However, HClO4 doped polyaniline shows the folded lamellar structure derived from the fibers. The thermal stability of these polymers was investigated with the help of thermogravimetric (TG/SDTA) analysis. The formation of a greater fraction of the conducting emeraldine salt phase is observed in HClO4 as a protonic acid media. The thermal stability of H3PO4 doped material is found to be better as compared with other acids. An increase in conductivity with an increase in temperature was observed in all the samples except for HClO4 doped polyaniline. Polym. Eng. Sci. 44:1676–1681, 2004. © 2004 Society of Plastics Engineers.

Mechanical response and rheological properties of polycarbonate layered-silicate nanocomposites

Abstract: The effect of clay loading on the mechanical behavior and melt state linear viscoelastic properties of intercalated polycarbonate (PC) nanocomposites was investigated. At low frequencies, the linear dynamic oscillatory moduli data revealed diminished frequency dependence with increasing nanoclay loading. The 3.5 and 5 wt% clay nanocomposites exhibited dramatically altered relaxation behavior, from liquid-like to pseudo-solid–like, compared to the pure PC and the 1.5 wt% clay nanocomposite. Thermal degradation of PC resulted from the melt compounding of organo-modified nanoclays was evident from the reduction in the glass transition temperature and molecular weight of the PC nanocomposites. These nanocomposites also exhibited a significant decrease in the extent of tensile elongation and ductility with respect to the nanoclay incorporation. A concomitant decrease in the rheological properties at high frequencies was also observed, and was consistent with the lowering of the molecular weight of PC, particularly near or above the percolation threshold of nanoclay. These nanocomposites, nevertheless, exhibited elastic-plastic deformation in compression, regardless of nanoclay content. Polym. Eng. Sci. 44:825–837, 2004. © 2004 Society of Plastics Engineers.

Mechanical and rheological studies on polyethylene terephthalate-montmorillonite nanocomposites

Abstract: In this work, a detailed investigation of the rheological and mechanical properties of nanocomposites based on polyethylene terephthalate (PET) and montmorillonite clays is presented. A series of additives [maleic anhydride (MAH), pentaerythritol (PENTA), and alkylammonium chlorides from amines of various chain lengths] were used as compatibilizers. The influence of the additives on the mechanical and rheological behavior of the PET matrix was evaluated separately, through their individual contributions. To shed more light on the reported decrease in melt viscosity of organoclay composites with respect to the matrix viscosity, observed in nylon and PET nanocomposites, the polymer molecular weight was related to the resulting rheological and mechanical properties of the systems. Results reveal that the PET nanocomposites behave quite differently in shear as opposed to elongation. The viscoelastic properties in shear present low values when the molecular weight decreases because of the processing operations. However, properties under tension increase over those of the matrix, manifesting the presence of a network formed between the clay and the polymer.

On the Effect of Pressure on the Shear and Elongational Viscosities of Polymer Melts

Abstract: The effect of pressure and temperature on the shear and elongational deformation rate-dependent viscosities has been experimentally investigated for several polymers (HDPE, LDPE, LLDPE, PP, PC, PMMA, and PS) on a capillary rheometer with a back-pressure device. Pressure, β, and temperature, α, coefficients have been determined through simultaneous fitting of the shear and extensional viscosity data by the modified White-Metzner model. The dependence of β and α on temperature and pressure, respectively, was investigated and it has been found that simple relationships exist between pressure and temperature sensitivity coefficients for individual polymers. © 2004 Society of Plastics Engineers.

Visual observation of CO2 foaming of polypropylene‐clay nanocomposites

Abstract: Using a newly developed high-pressure autoclave, which has two sapphire windows on the walls, we visually observed the batch physical foaming of polymer-clay nanocomposites to understand the effect of nano-sized clay on the initial stage of foaming. With CO 2 as a physical foaming agent, polypropylene-montmorillonite clay nanocomposites were foamed at 150°C. A high-speed digital camera with a microscope could observe the bubble nucleation and bubble growth behavior of the early stage of foaming in situ. The series of micrographs was analyzed in order to investigate the effect of clay content on bubble nucleation and growth. The experiments, together with CO 2 -solubility and diffusivity data, show that the clay enhances bubble nucleation as a nucleation agent and retards the growth of bubbles at the early stage of foaming.

Exfoliation and dispersion enhancement in polypropylene nanocomposites by in‐situ melt phase ultrasonication

Abstract: A novel process using ultrasonics to enhance the exfoliation and dispersion of clay platelets in polypropylene-based nanocomposites has been proposed and investigated. The materials studied were isotactic polypropylene of various molecular weights reinforced with organophilic montmorillonite clay (nanoclay) at 4–6 wt% loadings. X-ray diffraction (XRD) and rheological measurements, on a model system of nanoclay in mineral oil, were first used to determine ultrasonic energy requirements. The effectiveness of the proposed ultrasonic processing technique on polypropylene nanocomposites was evaluated by XRD and transmission electron microscopy (TEM). The effects of added maleic anhydride–grafted polypropylene compatibilizer, polypropylene molecular weight, and pretreatment of the nanoclays on the nanocomposite exfoliation were also investigated. Results indicate that ultrasonic processing of polymer nanocomposites in the melt state is an effective method for improving exfoliation and dispersion of nanoclays. Issues regarding molecular weight degradation, optimization, mechanical properties, and continuous processing are beyond the scope of the present study and are currently being investigated in our laboratory. Polym. Eng. Sci. 44:1773–1782, 2004. © 2004 Society of Plastics Engineers.

Analysis of the creep behavior of polypropylene-woodflour composites

Abstract: The creep behavior of composites prepared from woodflour and polypropylene has been analyzed. The woodflour content was varied from 0% to 60%. The compatibility between filler and matrix was varied by adding a non-commercial polypropylene-maleic anhydride copolymer (PPMAN) to the mixture. Short-term and long-term creep tests of woodflour/polypropylene composites at different temperatures were carried out. The effects of filler content, addition of compatibilizing agent and temperature were discussed. The creep deformation was generally reduced with woodflour addition, except at very high filler concentrations because of filler-wetting and dispersion problems. Low temperatures and addition of PPMAN also reduced the creep deformation. The creep compliance was modeled using the Burgers model and a power law equation. The parameters were found from the best fitting of experimental data using an optimization method. The Burgers model was found to provide a good description of the linear viscoelastic behavior. The mathematical description obtained from the short-term creep was utilized to predict the dynamic mechanical behavior of the composites with very good agreement between experimental and calculated values. Attempts to use the time-temperature-superposition principle to predict long-term creep from high temperature results were not successful because of the aging of the samples during creep at low temperatures. However, there was a good superposition of the short-term and long-term data at temperatures close to 70°C–80°C. The reason for this behavior is a relaxation of the PP matrix that takes place in that temperature range and erases any previous aging of the material. Polym. Eng. Sci. 44:1594–1603, 2004. © 2004 Society of Plastics Engineers.

Melt compounding of different grades of polystyrene with organoclay. Part 1: Compounding and characterization

Abstract: Nanocomposites of polystyrene (PS) with up to 20 wt% organoclay were prepared by melt compounding in a co-rotating, intermeshing twin-screw extruder. Three grades of PS with different molecular weights were used. In this paper we discuss preparation and characterization of the mixtures. Residence time and its distribution were measured by ultrasonics (US). They were found to be independent of the PS grade, but the US attenuation and sound velocity varied with the organoclay loading. According to the XRD diffraction data, the organoclay was dispersed into two types of platelet stacks evidenced by the diffraction peaks at about 2θ = 2.1 and 5.5°, i.e., with the interlayer spacings of d 001 = 4.20 and 1.6 nm, respectively. Since neat organoclay has d 001 = 1.93 nm, the first peak indicates intercalation by PS and the other a collapse of the interlamellar gallery during compounding. The XRD spectra depended on the organoclay content, but not on PS grade. According to TEM analysis, the degree of intercalation of organoclay in the polymer matrix is highest for the low-molecular-weight polystyrene. TEM results also confirm the collapse of interlamellar spacing in parts of the samples. FT-IR spectroscopy showed that there was some thermal degradation of the onium compound present in the nanoclay.

Structure‐property relationships in polymer blend nanocomposites

Abstract: Nanocomposites comprising nanoscale platelets derived from layered silicates treated with an organic modifier in thermoplastic vulcanizates (TPV) and PP/EPDM blends were prepared by direct melt intercalation. The interlayer spacing and dispersion of the nanoclay are greatly affected by polar forces between the nanoclay and the polymeric matrix material. The mechanical properties strongly depend on the structure and morphology of the nanocomposites, which can be modified by phase partitioning of the reinforcements. Morphology characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) provides the basis for understanding the observed structure-property relationships in this class of materials. With the increase of organoclay loading, the tensile modulus of TPV/clay nanocomposites increases by up to 170% at 8 wt% organoclay loading, while tensile strength gradually decreases with increase of organoclay loading. In the physical blend systems, the tensile modulus increases for all PP/EPDM blend compositions and generally shows higher values in the case of selectively reinforced phases in blends having a continuous PP matrix. The tensile strength of those blends decreases at higher nanoclay content no matter how blending is performed. However, the tensile strength may increase when sufficient selectively reinforced EPDM phase is present.

Rheological, mechanical and biodegradation studies on blends of thermoplastic starch and polycaprolactone

Abstract: Polycaprolactone (PCL) has been blended with thermoplastic starch (TPS), prepared from regular corn starch and glycerol, in a twin-screw extruder. The rheological, mechanical, thermal and morphological properties of the blends were examined. Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM) studies revealed that PCL/TPS blends are thermodynamically immiscible. However, they form compatible blends as a result of the hydrogen bonding interaction between the ester carbonyl of PCL and the OH groups on starch. Biodegradability of the blends increased with increasing TPS content. Dynamic viscoelastic measurements concluded that blends containing above 60-wt% TPS had higher storage and loss moduli than those of pure TPS and PCL. In addition, these blends had higher complex viscosities. Polym. Eng. Sci. 44:1429–1438, 2004. © 2004 Society of Plastics Engineers.

Morphology and performance of epoxy nanocomposites modified with organoclay and rubber

Abstract: Hybrid epoxy nanocomposites modified with carboxyl-terminated butadiene acrylonitrile (CTBN) rubber and organoclay were synthesized. The morphology of those nanocomposites was studied with X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The results showed that most of the organoclay in the nanocomposites was exfoliated and high cure temperatures benefit the formation of exfoliated nanocomposites. Organoclay simultaneously improved fracture toughness, compressive modulus, yield strength and ultimate strength of the epoxy resin. Organoclay not only enhanced T g , yield strength and ultimate strength of rubber-modified epoxies, but also further improved their fracture toughness. There was a superposition effect on fracture toughness of the hybrid epoxy nanocomposites modified with both rubber and organoclay.

A local theory of heating in cross‐ply carbon fiber thermoplastic composites by magnetic induction

Abstract: For joining and repair of continuous fiber thermoplastic composites, induction heating has been viewed a strong candidate. Induction heating employs an applied alternating magnetic field, which induces a rotational emf in a grid of conductive carbon fibers, which are then used to carry resulting currents. In continuous carbon fiber crossply composites the available paths for “eddy current” loops are along the network of conductive carbon fibers. For this to occur, an electrical transfer must take place between crossing fibers in adjacent plies. Tests involving variable thicknesses of interply neat film layers have been performed to provide insight into the mechanisms taking place. These tests indicate that the primary mechanism for heating in such laminates is dielectric losses in the polymeric region between fibers in adjacent planes that form the conductive loop. Therefore, heating is not uniform in such composites despite a uniform magnetic flux. Heating patterns were viewed using liquid crystal materials and E-type thermocouples. Several factors leading to nonhomogeneous thermal distributions have been considered, including current density effects, internal emf cancellation, and rotational field effects. Global and local considerations are addressed, a localized model is proposed, and the corresponding theory is developed qualifying the early results. Additional testing has supported the theory.

Curing kinetics and chemorheological analysis of polyurethane formation

Abstract: The cross-linking kinetics and the rheological behavior of a polyurethane system during curing are investigated by DSC and small amplitude oscillatory shear rheometry. In this study, isoconversional methods are applied to understand the reaction complexity. The activation energy depends on the extent of conversion. It can be shown that three parallel reactions occur during the evolution of the global reaction in non-isothermal conditions. An original multi-step model can involve these three parallel independent reactions. Dynamic viscosity is measured as a function of time and is found independent of shear rate. Isothermal viscosity rise is related to reaction extent by a phenomenological model. The gelation time is determined by the tanδ crossover at different frequencies. Polym. Eng. Sci. 44:518–527, 2004. © 2004 Society of Plastics Engineers.