Melt Intercalation in Montmorillonite/Polystyrene Nanocomposites

Atactic polystyrene (PS) was used to study the effect of flow field (shear and/or elongational) on the intercalation of polymer/clay nanocomposites (PNC). Three grades of (PS), with different molecular weights, were compounded with an ammonium-modified montmorillonite (Cloisite 10A) in a twin-screw extruder (TSE). The compounds were subsequently fed to a single screw extruder, fitted with one of three specially designed torpedo-attachments. The attachments were designed to provide combinations of different levels of shear and elongational deformations. The resins, TSE compounds, and final PNC’s were characterized for the degree of intercalation, degradation, rheological behavior, and mechanical properties. The data showed that the thermal decomposition of the quaternary ammonium intercalant caused severe damage to both PNC components: a collapse of the organoclay interlayer spacing, and the thermo-oxidative degradation of PS. In spite of these detrimental effects, the attachment employing combined elongational and shear flow resulted in generally larger gallery spacing and more improvement of the mechanical properties than the other two attachments.

Simultaneous Biaxial Stretching of Polypropylene Resins of Varying Tacticity

The simultaneous biaxial stretching behavior of three Ziegler-Natta-based isotactic polypropylene resins of varying isotacticity was studied using a laboratory film stretcher that simulates closely stretching conditions encountered in the commercial process. The effect of isotacticity on the final morphology and end film properties was associated with the effect of isotacticity on the thermal properties of the resin. The results showed that the degree of undermelting, △Tum = Tm–T, determines the phase composition of the polymer before stretching and the morphology and orientation of the stretched films. The size and orientation of the crystallites in the stretching plane is mostly determined by the degree of undermelting. The elongation at break was correlated with the residual crystallinity of the film before stretching, and the elastic modulus was correlated with the final degree of crystallinity and the density of tie chains.

Morphological Characterization of PE Blown Films by Atomic Force Microscopy

The properties of blown polyethylene (PE) films depend on various factors, including crystallinity, morphology, and orientation, in addition to chemical composition. It has been shown that the optical properties are strongly influenced by surface morphology. In this work, non-contact atomic force microscopy (AFM) and polarized light microscopy (PLM) were used to visualize surface and bulk morphology. Various techniques, such as surface and line roughness, surface and line fractal dimension, pair-correlation function and nearest neighbor distance distribution function, are employed to quantify the description of morphology and to compare the morphological characteristics of a number of polyethylene (PE) films of commercial interest. A comprehensive quantitative analysis of surface topography has been performed. The co-monomer of the PE resins was found to play a significant role in the formation and the orientation of spherulite-like domains. The film cross-section microstructure has been evaluated qualitatively by using both AFM and PLM. However, quantitative analysis of bulk morphology cannot be obtained due to knife effects.

Electrical, Morphological and Thermal Properties of Microinjection Molded Polypropylene/Multi-Walled Carbon Nanotubes Nanocomposites

A series of polypropylene/multi-walled carbon nanotubes (PP/CNT) nanocomposites were prepared by masterbatch dilution, then followed by microinjection molding under a defined set of processing conditions. A micropart (μ-part) which has a three-step decrease in thickness along the flow direction was fabricated to study the effect of abrupt geometrical changes in mold cavities on the distribution of CNT in PP. To facilitate characterization, the μ-parts were divided into three sections based on thickness. The distribution of CNT within each section of subsequent μ-parts was evaluated by morphological observations and electrical resistivity measurements. In addition, the thermal properties of pure PP and PP/CNT nanocomposites as well as each section of subsequent μ-parts, were assessed by differential scanning calorimetry and thermogravimetric analysis.

The Effect of Cellulose Nanocrystals (CNC) on Isothermal Crystallization Kinetics of LLDPE and HDPE

Highly porous agglomerates of spray freeze dried cellulose nanocrystals (SFD-CNC) were prepared, starting with sonicated aqueous suspensions of spray-dried cellulose nanocrystals powder (SD-CNC). Subsequently, SFD-CNC together with the SD-CNC (used as a reference) were incorporated into LLDPE and HDPE via melt compounding in a batch mixer to produce nanocomposites containing 0.5 wt.° and 2 wt.° CNC. Differential scanning calorimetry (DSC) was used to study the thermal properties and the isothermal crystallization kinetics of the polyethylenes and the nanocomposites. Polarized light microscopy (PLM) was used to evaluate the growth kinetics and spherulitic structure of polyethylene in both the filled and unfilled polymers. Avrami crystallization kinetics models were employed to analyze the DSC results. It was observed that CNC acts as a heterogeneous nucleating agent in LLDPE nanocomposites, thus yielding nucleation controlled crystallization. On the other hand, in the HDPE systems (polymer and nanocomposites) heterogeneous nucleation was followed by 3-D growth. It was observed that CNC slightly hindered the formation of chain folding for the HDPE, similar to previous studies on the polypropylene and its nanocomposites. Spray freeze drying produced twice as many nucleation sites compared to spray dried samples and it enhanced the overall crystallization rate and the crystallinity.

Flow and Thermal History Effects on Morphology and Tensile Behavior of Poly(oxymethylene) Micro Injection Molded Parts

The micro injection molding process is a rapidly growing area in plastics processing technology. In this process, the polymer is exposed to both high shear rates and large thermal gradients. In view of the versatility of the process, both commodity and engineering polymers have been used in micro injection molded products. In the present work, poly(oxymethylene) (POM), a partially crystalline engineering polymer, was employed to evaluate the relationships between processing conditions, on one hand, and the morphology and properties of the final part, on the other hand. An unsymmetrical mold cavity to make parts in the form of stepped plaques was used in the study. This resulted in substantial differences in morphology, crystallinity and shrinkage of the zones of different constant thicknesses in the micro parts. Depending on the molding conditions and the location on the micro-part, the microstructure can display up to five crystalline layers. Of particular interest, shish-kebab crystalline structures were observed within the skin of the step with the smallest thickness. Differential scanning calorimetry (DSC) tests are used to distinguish between the melting points of the shish and kebab components of this particular structure. The degree of crystallinity as determined by wide angle X-ray diffraction (WAXD) and shrinkage across the thickness were also found to be highest in the step with the smallest thickness.

The Effect of Molecular Parameters on the Thermal Behavior of Recycled and Virgin Polyamides and Their Glass Fiber Composites

A study was carried out to evaluate the effects of molecular weight and molecular structure on non-isothermal crystallization behavior of recycled and virgin polyamides (PA) and their corresponding glass fiber (GF) composites. Two different recycled polyamides (PA), namely post-industrial waste (PIW) and post-consumer waste (PCW) were used. The former was obtained from a fiber manufacturer and the latter was recycled from used carpets. The molecular weights of the resins were measured by intrinsic viscosity (IV) measurements and 13carbon nuclear magnetic resonance (13C-NMR). The NMR technique also provided information on PA structure, cis and trans amide conformers content and residual unreacted monomer. Non-isothermal crystallization of the resins was studied using differential scanning calorimetry (DSC). The molecular weights of recycled materials were higher than that of virgin injection molding grade PA-6. However, the crystallization rates (indicated by t1/2) of recycled resins were faster. It could be attributed to the presence of TiO2 in recycled materials. Moreover, the higher cis-amide conformer content of recycled resins suggested higher segmental mobility. On the other hand, the crystallization rate of composites based on recycled PA-6 was slower than that of composites based on the virgin PA-6. The suppression of crystallization rate was apparently due to PA-66 added during the preparation of reinforced recycled resins. This behavior was confirmed by model compounds based on PA-6/PA-66 blends.

Effect of surface energy on dispersion and mechanical properties of polymer/nanocrystalline cellulose nanocomposites

Dispersion quality and polymer-filler interaction are important factors in determining the final properties of polymer nanocomposites. Surface energy of nanocrystalline cellulose (NCC) and some polymers (polypropylene, PP, and polylactic acid, PLA) was measured at room and high temperatures. NCC had higher polarity and surface energy than PP and PLA at room temperature but had a lower surface energy at higher temperatures. The effect of surface modification with alkenyl succinic anhydride (ASA) on NCC surface energy at room and high temperature was studied. Total surface energy of NCC was lowered after surface modification. Thermodynamic work of adhesion for PP/NCC and PLA/NCC was lowered by NCC surface modification. A thermodynamic analysis is proposed to estimate the dispersion energy, based on surface energy measurements at room and high temperatures. Also, a dispersion factor is defined to provide a quantitative indication of the dispersibility of nanoparticles in a polymer matrix under various conditions. The required dispersion energy was reduced by lowering the interfacial tension. On the other hand, it increased as the quality of NCC dispersion (i.e., the nanoparticle surface area) in the system was improved. Surface modification of NCC with ASA had a negative effect on the compatibility between NCC and PLA, whereas it had a positive influence on compatibility between PP and NCC.

Measurement and Prediction of Temperature Distribution in an Injection Molding Cavity

A method to measure the melt temperature distribution in an injection mold cavity is developed. A thermocouple is used in the construction of a sensor with a tip that can be adjusted at different depths of a mold cavity, to measure temperature profiles at different positions in the mold. Polymer temperature distributions are measured and factors affecting temperatures and the key variables that influence distributions are determined. Measurements are compared with temperatures obtained from numerical simulation of the injection molding process using a three-dimensional heat transfer analysis. Although showing lower values and generally higher cooling rates, temperature data measured from the mold cavity indicate similar behavior to predicted transient temperature distributions.

The Pressure-Volume-Temperature Behavior Polyethylene Melts

The pressure-volume-temperature (PVT) melt behavior of 12 polyethylene resins was evaluated at pressures up to 200 MPa, using both isothermal and isobaric measurements in a GNOMIX high pressure dilatometer. The resins included high density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). They were produced using a variety of catalysts, including Ziegler-Natta (ZN) and metallocene catalysts. The PVT data were used to evaluate two empirical equations of state (the Tate and Inverse Volume equations) in predicting PVT behavior of the melt, the isothermal compressibility, and the thermal expansion coefficient. The dependence of the melting and crystallization temperatures on pressure was also evaluated, and compared to existing equations. It was not possible to identify structural effects on the PVT melt behavior, as well as the isothermal compressibility and thermal expansion coefficient. However, some slight dependence on density was observed for the parameters of the equations of state. The isobaric experiments revealed that the pressure effect on the melting and crystallization temperatures was similar for all resins. While the melting and crystallization temperatures varied widely for the various resins, the pressure coefficients of the melting and crystallization temperatures were approximately equal (0.25°C/MPa.) The main differences in the behavior of the resins were in the transitional region during melting and crystallization.

Effect of Temperature and Compatibilizer on Interfacial Tension of PE/PA-6 and PP/EVOH

Interfacial tension measurements for polyethylene/polyamide-6 (PE/PA-6) and polypropylene/ethylene-vinyl alcohol copolymer (PP/EVOH) pairs were performed using the spinning drop method. The effects of the temperature and maleation content of the polyolefin on the interfacial tension were evaluated. The temperature coefficients of interfacial tension were found to be 0.036 and 0.087 dyne/cm/°C for the PE/PA-6 and maleated PP/EVOH systems, respectively. The interfacial tension of PE/PA-6 system showed exponential dependence on the maleation content of PE. The accuracy of the spinning drop method was evaluated against the pendant drop technique. It was found that the spinning drop technique tends to yield lower interfacial tension values than the pendant drop method. The harmonicmean equation was found to provide a fair estimate for the interfacial tension of PE/PA-6 pair.

Physically-Based Adaptive Control of Cavity Pressure in Injection Molding: Filling Phase

Cavity pressure plays an important role in determining the quality of injection molded articles. The non-linear and time-varying nature of the process causes difficulties in implementing efficient control systems, which usually employ linear, time-invariant design strategies. Conversely, advanced control methods, such as self-tuning control, do not provide physical information about the process. A physically-based adaptive controller has been developed for cavity pressure control during the filling phase. Different control algorithms, employing different tuning criteria, were implemented on a microcomputer to control the cavity pressure. Experimental results indicate that this control yields accurate results for the injection molding of polyethylene. The results also show that this method, using the IMC algorithm, is superior to the classical optimal controller tuning approach.

Inverse Modeling of Injection Molding Thermal Stresses to Optimize Temperature and Pressure History

The dimensional stability and mechanical and optical properties of injection molded articles are affected to a large extent by the distribution of residual stresses. The distribution of residual stresses varies as a result of the temperature and pressure history during the process. In this paper, two thermoelastic models were used to study the effect of processing conditions on generation of thermal stresses. Then, based on these models, two inverse models were developed to calculate the unknown pressure history and initial melt temperature from a known residual stress distribution in the final product. The models were successfully tested with stress data simulated by the forward models. The effect of experimental errors on the inverse methods was investigated by adding random errors to the stress data. Finally, experimentally measured stress profiles were used to further test the inverse models. The results are presented and discussed.

Physically-Based Model of Thermoplastics Injection Molding for Control Applications

A non-linear mathematical model of the thermoplastics injection molding cycle is presented. The model is formulated based on the conservation equations for the filling, packing, and cooling phases. The whole process is divided into subsystems including the hydraulic system, ram-screw, barrel, and polymer delivery system. It is found necessary to account for polymer melt elasticity as well as the non-Newtonian behavior of the polymer melt flow. The growing solid skin in the sprue, runner, and gate are considered. The governing equations are solved numerically. Model predictions are in good agreement with experimental data for the injection molding of high density polyethylene. The resulting model is a useful tool for the study and design of injection molding controllers, machine parameter selection, and equipment design.

Dynamics and Control of Surface and Mold Temperatures in Injection Molding

Mold temperature control in injection molding is very important for control of production rate and product quality. However, mold temperature is a complex function of machine design, process variables, machine settings and ambient conditions. Data were obtained regarding the variability in both space and time (in-cycle and cycle-to-cycle) of mold metal and mold surface temperatures and heat flux. In view of this variability, it is necessary to define variables which may be useful for control purposes. The following variables were evaluated: cycle average temperature, peak temperature, and partial cooling time. Dynamic models were developed, on the basis of experimental data, to determine the process gains, time constants and time delays for the above variables with regard to coolant temperature, coolant flowrate, and melt temperature as the possible manipulated variables. In view of the dynamic studies, the cycle average surface temperature was selected as the controlled variable for control system design and simulation. The regulating abilities of PI, PID, and Dahlin controllers were evaluated. The results indicate that Dahlin control caused the controlled variable to follow the set-point closely, even when the process parameters vary, thus showing good robustness. However, the Dahlin algorithm achieves this advantage via large variations in the manipulated variable. This may cause difficulty in some circumstances.

Closed Loop Control of Parison Dimension Profiles in Extrusion Blow Molding

Extrusion blow molding is utilized in the production of plastic hollow containers. One of the critical steps in the process sequence is the formation of an annular extrudate, called the parison. Extrusion of the parison through an annular die is associated with varying degrees of swell and sag along the length. Moreover, the design of many commercial blow molding products requires variations in wall thickness along the length of the parison. The present work describes the design and operation of an on-line computerized parison thickness profile measurement technique for the purpose of implementing closed loop control of the parison thickness profile.

The proposed thickness measurement technique was evaluated by comparing it to the conventional pinch-off mold technique. The control strategy employed involves a simple on-line adaptive control scheme. Programmed set point profiles, such as constant, linear and oscillatory profiles were implemented to within a 5% error. Moreover, the adaptive controller compensates for system disturbances.

Simultaneous Biaxial Stretching of PP Resins of Varying Tacticity

The melting and simultaneous biaxial stretching behavior of three Ziegler-Natta isotactic polypropylene resins of varying isotacticity was studied. DSC melting experiments showed that the crystallinity and melt stability of the crystalline phase depend strongly on tacticity and on the degree of undermelting, ΔT = Tm – T, where Tm is the melting temperature. The level of biaxial yield stress depends on the fraction of residual “unmolten” crystallinity and the crystallite size. For specimens with same thermal history, the post-yield deformation shows strong dependence on the degree of undermelting, ΔT. The deformation homogeneity and the resulting film thickness may be related to the thermal mobility of the chains and the tie chain and entanglement densities.

Non-linear Crystalline Spherulitic Growth Behavior for LLDPE

Nonlinear growth behavior was observed in two crystallization regimes, depending on the temperature. Non-linearity may be explained by the reduction of the concentration of crystallizable ethylene sequences (CES) in the melt phase. In the two regimes, the concentration of uncrystallizable ethylene sequences (UCES) increases, as the crystallization time increases, because UCES are continuously excluded from the crystal lattice into the melt phase. An empirical equation is proposed to describe the melting temperature of the crystal stem with the maximum possible length, T C, n * m , in nonlinear growth processes, assuming that the diffusion layer is negligible. A modified form of the Hoffman-Lauritzen equation (MHL) describes well the crystallization growth kinetics of LLDPE spherulites in the non-linear growth region.

Cystic schwannoma of the pelvis

Schwannomas are benign tumours that arise from the Schwann cells of nerve fibres. They commonly occur in the head and neck, mediastinum and extremities. They are extremely rare in the pelvis. These are usually slow-growing tumours and are often detected incidentally. Preoperative diagnosis is extremely difficult as there are no definitive signs on imaging. Aspiration biopsy is often inconclusive or misleading. Surgical excision is both diagnostic and therapeutic. As these tumours are often large in size, open excision is most commonly performed. We describe a case of a large, cystic schwannoma of the pelvis causing bladder outlet obstruction and bilateral hydroureteronephrosis. Complete surgical excision was performed laparoscopically.

Characterization of Thermoplastic Laser-welded Joints

Laser-welded joints were prepared using moldings of polycarbonate, polyamide-6, and polyamide-6 reinforced with 30% glass fibers. Preparation of thin polished samples of the joints facilitated examination of the laser-affected zone (LAZ). The samples were examined using optical and polarized light microscopy, as well as scanning electron microscopy. The study evaluated the influence of laser power on the geometry and dimensions of LAZ, in both the absorbent and non-absorbent parts of the joints. The results indicated a correlation between LAZ dimensions and the tensile strength of the laser-welded joints. There is evidence of melt and fiber migration in the LAZ.

Effect of Processing Conditions on Properties of PET/Clay Nanocomposite Films

Polyethylene terephthalate (PET) nanocomposite films (with 3 wt.% Cloisite 30B) were prepared by cast extrusion followed by uniaxial stretching, using chill rolls. Two screw profiles with different mixing elements under different screw speeds (N) and feeding rates (Q) were used to prepare PET/clay nanocomposite (PCN) films. Transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD) showed that the clay layers were aligned in the machine direction (MD). XRD patterns depicted that the interlayer distance of clay platelets in the state of intercalation is somehow independent of the processing conditions, but the macro-scale characterization, including barrier and mechanical properties, showed that the level of clay layer delamination was affected by processing conditions. The results reveal that the applied strain has stronger effect than residence time on the barrier and mechanical properties. At the highest screw speed (N = 250 min−1), 27% reduction in oxygen permeability and 30% improvement in tensile modulus were obtained for the more severe screw profile.

Polystyrene/Phosphonium Organoclay Nanocomposites by Melt Compounding

Polystyrene-montmorillonite nanocomposites were prepared by melt compounding, using several ammonium and phosphonium organoclays. Melt processing was carried out in a twin screw extrusion system, specially modified to produce improved dispersion and longer residence time. The effect of molecular weight of polystyrene on clay dispersion and property enhancement was evaluated. Nanocomposite structure was characterized by wide angle x-ray diffraction (WAXD) and transmission electron microscopy (TEM). Thermal stability and mechanical and barrier properties were also determined. The quality of dispersion of organically modified montmorillonite depended on the molecular weight of the polystyrene resin. Barrier properties were measured and compared to predictions of permeability models available in the literature. Clay dispersion and property enhancement were explained in relation to the surface characteristics of the organoclays, and the work of adhesion at the polystyrene-clay interface was correlated with the tensile modulus of the nanocomposites.

Micro Replication by Injection-Compression Molding

The replication of polymeric microstructures on a flat surface is not easily achieved by conventional injection molding, because of the required micro-dimensional control and the stringent tolerances of most applications. Moreover, the flow behaviour of melts in micro cavities remains not well understood and challenging. On the other hand, injection-compression molding is an established process for the manufacturing of optical storage media, like compact discs (CD) or digital versatile discs (DVD), with grooves and pits at the micro-scale. The difficulties arise mainly from two sources: lack of adequate filling because of premature freezing of features with very small thicknesses and excessive deformation during ejection due to high friction at the polymer-metal interface. In this work, a study is carried out to investigate the effect of process parameters on the replication of various microstructures on a flat disk using microinjection-compression molding. A commercial microinjection molding machine has been used in the study. Two optical grade polymers poly(methyl methacrylate) (PMMA) and cyclic olefin polymer (COP) were used in conjunction with various mold inserts to reproduce the embedded microstructures. The dimensions of the microstructures on the inserts and molded parts were measured with a confocal profiler. A transcription ratio was defined to assess the quality of the replication. The design of experiment (DOE) method was used to obtain correlations between the process parameters and the development of the microstructure during injection-compression molding.

Melting Temperature Characteristics for Polyethylenes from Crystal Size Distribution

Semi-crystalline polymers exhibit broad and multiple peaks in their melting traces. Thus, the melting temperature characteristics of polymers should consider both melting peak positions and melting temperature polydispersity. In this work, the effective melting temperature and temperature polydispersity are defined and calculated from DSC traces, using the crystal size number distribution and the melting temperature equation. Three methods are proposed for calculating melting temperature characteristics. These methods are based on: (i) average crystal size, (ii) the crystal stem number distribution function, and (iii) the monomer structural unit distribution function. They were employed to analyze the isothermal and non-isothermal experimental results for polyethylene polymers, especially linear low-density polyethylene copolymers. The first method, based on the value of average crystal size, gives the most reasonable results, taking into consideration agreement with experimental observations and structural data.

Phase Separation under Shear in Liquid Crystalline Polymer/Polycarbonate Blends

The effect of low steady shear rates on the phase diagram and phase-separated morphology of liquid crystalline polymer (LCP)/polycarbonate (PC) blends was studied. Experiments were carried out with polarized light microscopy in conjunction with a shear stage for directly monitoring the phase separation behavior of the blends. Phase separation temperatures under shear flow were lower than those observed under quiescent conditions. Consequently, the phase diagram of the blend was shifted to a lower position, relative to the phase diagram obtained under quiescent conditions. The temporal morphological development of phase separation in the blends was also affected by the shear. Phase-separated structures were monitored, with and without shear, in blends containing 50 wt.% LCP. Results showed that the speed and amount of phase separation increased when shear was applied. The results indicate that these blends undergo shear-enhanced phase separation at low steady shear rates.

Numerical simulation of polymer nanocomposites using self-consistent mean-field model

Clay-containing polymeric nanocomposites (PNC) are mixtures of dispersed clay platelets in a polymeric matrix. These materials show enhancement of physical properties, such as modulus, strength, and dimensional stability, as well as a reduction of gas permeability and flammability. The performance is related to the degree of clay dispersion (i.e., intercalation or exfoliation) and the bonding between the clay and the matrix. The main goal of this work has been to map the degree of dispersion as a function of independent variables (viz., magnitude of the interaction parameters, molecular weights, composition, etc.). In this paper, we present the results of the numerical analysis of the equilibrium thermodynamic miscibility using one- and two-dimensional (1D and 2D) models based on the self-consistent mean-field theory. In the limit, the 2D model reproduced the 1D model published results. The adopted 2D model considers the presence of four PNC components: solid clay platelets, low molecular weight intercalant, polymeric matrix, and end-functionalized compatibilizer. The simulations, with realistic values of the binary interaction parameters, were analyzed for potential exfoliation of PNC with a polyolefin as the matrix. The simulation results show that intercalation and exfoliation is expected within limited ranges of the independent variables. The presence of a bare clay surface (e.g., generated by thermal decomposition of intercalant or extraction by molten polymer) has a strong negative effect on the dispersion process. The simulation successfully identified the most influential factors, e.g., optimum ranges of the compatibilizer and the intercalant concentration.