محمدحسین نویدفامیلی

Hypothesis: 

One of the existing challenges in the foam production industry is to achieve the desired mechanical and thermal insulation properties required, which are directly related to cellular density, size, and distribution of bubbles. Therefore, predicting the bubble size distribution in each foam production system significantly contributes to the final properties of the desired foam. Nucleation, growth, coalescence of bubbles, and their final stabilization are influential stages in the ultimate properties of the foam that should be considered in predicting the bubble size distribution and laboratory testing phase.

Initially, a modified classical nucleation model was used for predicting cell nucleation, and then a population balance model was employed to predict the size distribution of bubbles in a batch system for the production of polystyrene foam. The modeling of this process was one-dimensional, and changes in bubble diameter were included as the characteristic variable of the system's equations. The foam production stage was carried out at temperatures of 70°C, 90°C, and 110°C, under a pressure of 20 MPa, and the consolidation stage was performed within 0.1 s and 1 s and without consolidation. To calculate the average cell size and size distribution, SEM images and software tools such as Axiovision.v4.82.SP2 and SPSS 26 were used, and the results were compared with the modeling outcomes.

Using the bubble size distribution obtained from modeling, the average bubble size at a saturation temperature of 70°C and a consolidation time of 1 s was 4.3 µm. With an increase in temperature from 70°C to 90°C, the average bubble size increased to 36.7 µm due to the higher rate of gas diffusion into the bubbles. With an increase in the amount of gas in polystyrene, the free volume increased, and glass transition temperature decreased. At 110°C, the average size of bubble cells increased to 78.1 µm. Since this temperature was higher than the glass transition temperature of polystyrene, in addition to the high gas diffusion rate into the bubble cells, the growth process did not stop, and gas diffusion and coalescence between the bubbles continued. Finally, the model predictions were compared with experiments under various conditions and demonstrated acceptable agreement.

The morphological attributes of polymer blends have a significant influence on the final properties of the blend. Many attempts have been made to simulate morphological changes of a blend in various equipments, using several mathematical models and numerical simulations. In this study, mixing of non-Newtonian fluids was simulated using a power-law rheological model in a typical twin screw extruder in three dimensions. Modified marker and cell (MAC) method was used for simulation. In modified MAC, for simulation of polymeric fluids flow, power law rheological model was used to generalize velocity equations to non-Newtonian fluids. Interfacial tension and viscosity ratios were used to introduce the effect of miscibility parameter. In this method, momentum equations were solved and discredited using finite volume method. The linear algebraic equations were solved using three diagonal matrix and the coefficients of network’s points were obtained. Then, velocity values were obtained for any control volume in three dimensions by introducing typical rheological and interfacial tension parameters for a pair of polymers. The under relaxation factors (URFs) were used for velocities in three dimensions and pressure for faster convergence. Then, new spatial peculiarities of particles of two phases were traced by taking appropriate time intervals. The geometry of screws in twin screw extruder was achieved using hypothesis of imaginary domain of screws tips. A twin screw extruder with glass barrel was designed and constructed for intuitive investigation of mixing development. Mixing of polymer pairs of different miscibility was performed in equal time intervals. The experimental pictures were compared with the simulation results. A very good agreement was observed between the results of numerical simulation and experimental images in both miscible and immiscible fluids. The determined separation areas for theoretical and experimental images also confirmed the accuracy of the simulation. Therefore, modified marker and cell (MAC) method could be used to simulate the mixing of polymer blends in twin screw extruder, as the most common blending equipment.

Hypothesis: The dispersion state of nanoparticles cannot be evaluated accurately by rheological and mechanical testes because their agglomerates break down under applied stress leading to changes in their microstructure. Additionally, the contribution of interparticle interactions to macroscopic properties such as rheological and mechanical properties is still unclear. This is mainly due to the combined effects of interparticle interactions and the particle-polymer interactions. Microscopic measurements which can only provide information on a very small section of a sample are not reliable for this purpose. However, electrical tests as nondestructive methods can be used to assess the microstructure and cluster formation in nanocomposite structures. In addition, it is possible to detect separately the effect of filler-filler interactions using dielectric properties and with a proper choice of materials.
In this study, ZnO/polystyrene (PS) nanocomposites were prepared through conventional mixing, dispersive mixing and surface particle treatment to control the particle dispersion states. The dispersion state of nanoparticles has been analyzed using their optical, electrical and rheological properties.
Findings: The results showed that storage modulus increased with increasing filler dispersion, which could be attributed to the increase in interfacial layer and the higher modulus of nanocomposite relative to that of the bulk polymer matrix. The SEM images showed that the dispersive mixing and surface treatment of ZnO nanoparticles with oleic acid improved the dispersion of ZnO particles inside the PS matrix. On the other hand, increase in dispersion decreases the electrical conductivity and dielectric constant due to greater interparticle distance and reduction of dipole-dipole interactions, respectively. Hence, it is possible to detect the particle dispersion state by rheological and electrical properties.

Both physical and chemical foaming agents can be used in the two common methods for the production of polymeric foams. The physical foaming agent is dissolved in the polymer at a pressure and temperature beyond its critical condition. The foam structure is formed by a sudden pressure drop in the mixture in three steps; nucleation, growth, and coalescence. In the nucleation step, thermodynamic instability is formed in the mixture. This induces a tendency in the solvent molecules for a phase transition from a supersaturated state to a gas state in the direction of instability reduction. By overcoming the energy barrier, the free energy of the system is reduced and gas stable nuclei are formed in the free volumes of the polymer chains. Growth and coalescence of the nucleic take place with the diffusion of the gas molecules inside them. The growth step is finally stopped and the foam structure is established. With the increase in the nucleation efficiency, the growth rate is reduced. Therefore, prediction of nucleation rate is an important factor for controlling the thermoplastic foam structures. Investigation of the nucleation step has been doable with the aid of nucleation theories. The classical nucleation theory is a prominent method for the investigation of nucleation phenomena in thermoplastic foams, but due to the existing of a divergence between its theoretical values with the experimental data, some modifications have been introduced into this theory. Other competing theories such as the Density Functional Theory and the Self Consistent Field Theory have also been developed. The main aim of this review paper is an investigation of the existing theories for thermoplastic foams.

Microcellular thermoplastic polyurethane foams are examined as absorbing materials in the X-band (8.2-12.4 GHz) frequency range by means of experiment. In this work, we aim to establish relationships between foam morphology including cell size and air volume fraction and electromagnetic properties including absorption, transmission and reflection quality. Nanocomposites based on thermoplastic polyurethane containing carbon black were prepared by coagulation method. In this procedure 15 wt% carbon black-containing nanocomposite was converted to microcellular foams using batch foaming process and supercritical carbon dioxide as physical foaming agent. The morphology of the foams was evaluated by scanning electron microscopy. S-parameters of the samples were measured by a vector network analyzer (VNA) and the effect of morphological parameters such as cell size and air volume fraction on the absorbing properties was investigated. We also established structure/properties relationships which were essential for further optimizations of the materials used in the construction of radar absorbing composites. Foaming reduced the percolation threshold of the nanocomposites due to the reduction in the average distance between nanoparticles. Foaming and dielectric constant reduction dropped the reflection percentage significantly. The increase in air volume fraction in the foam increased absorption per its weight, because of multiple scattering in composite media. The sensitivity of electromagnetic wave toward the variation of cell size is strongly weaker than that toward the variation of air volume fraction. Electromagnetic properties of the microcellular foams deviated a little from effective medium theories (EMTs). Air volume fraction of the cells was a function of cell size and smaller cells showed higher absorption.

The components’ content of foams, based on thermoplastic/thermoset blends, was studied by reaction extrusion method. For this purpose, a co-rotating twin screw extruder was designed and assembled. A high pressure-high temperature vessel was used in which the pressure could be dropped quickly. The foaming was carried out at saturation pressure of 68 bar in 2 s at 170˚C. The results showed that the poly(vinyl chloride)/polyurethane (100/0) sample absorbed up to 8.09 % CO2 gas and produced foams with a cell average of 6 μm and a cell density of 6×109. In the blend ratios of 90/10, 80/20, 70/30, 60/40 and 50/50, up to 4% by weight of CO2 gas was absorbed and no suitable foams were produced. Since the amount of gas diffusivity in the polyurethane was more than that in poly(vinyl chloride), the amount of gas leakage was equally increased by increases in polyurethane content, therefore there was no sufficient absorbed gas available for foam formation. It should be noted that the swelling caused by gas evolution is reduced due to the cross-linking structure which is highly resistant against the volume expansion. During the foaming step, the cross-linking density should be high enough to prevent viscous flow failure but not to the extent to initiate brittle failure in expanding bubbles. Therefore, it can be perceived that the optimum crosslinking density associated with the maximum foam expansion ratio could be achieved when the other processing parameters were fixed.

This study addresses the effect of temperature and nanoparticle on PS foam structure in order to control its structure more accurately. For this purpose, a theoretical hypothesis was proposed by explaining the classical nucleation theory. The PS in the presence of nanosilica and CO2 was foamed. Foaming process was carried out in a vessel suitable under high pressure and temperature conditions, and with instantaneous pressure release and high-speed stabilization capabilities. The most important factors affecting foam properties including foaming temperature, size, content and surface properties of nanosilica were investigated. Increasing of foaming temperature was effective on the initial nuclei formation and cell growth. These two effects determined the final foam structure. When the temperature was changed from 90 to 180°C, cell density of PS foam increased thousand fold to 2.2×1012 number of cells per unit volume of foam (cell/cm3). The results showed that a small amount of nanosilica had a substantial effect on decreasing the cell size and increasing the cell density. An increase in nanoparticle concentration also increased its effectiveness. Moreover, the quality and structure of foam were improved by adding the nanoparticle. As the size of nanosilica increased from 20 to 40 nm, its cell density decreased from 3.3×109 to 1.78×109 numbers of cells per unit volume of foam (cells/cm3). Surface treatment of the nanosilica using triethoxysilane, in addition to improving nanoparticle dispersion, increased its cell density. The efficiency of nanosilica in improving cell density after surface treatment increased by more than double.

Microcellular thermoplastic foams can be usually produced in a one-step batch system using a physical foaming agent which is dissolved in a polymer system under specific pressure and temperature, higher than the critical condition of solvent and the glass transition temperature of polymer and solvent mixture. By application of a sudden pressure drop the foam structure is formed through stages of nucleation, growth and coalescence. After pressure drop, if the foam temperature is reduced below the glass transition of the gas-polymer mixture, the cells stop growing which results in a foam with stabilized morphology. This stabilization stage has not been thoroughly focused in previous studies. In this work, polystyrene as a polymer system and supercritical carbon dioxide as a solvent were used at 18.5 MPa pressure and different temperatures. The stabilization process took place within milliseconds and helped to a better understanding of cellular structure in thermoplastic foams. In this mechanism, the nucleation takes place in the phase transition of solvent molecules at supercritical state to the gas state and the formation of very small nuclei containing gas molecules between polymer chains. The energy originated from the nuclei growth is in competition with the elastic energy of polymer chains, and the predominance of one type of energy over another determines the final cell size. The results showed that the effect of stabilization process on the structure of the foam depended on the foaming temperature. Stabilization at 110°C resulted in a 50% cell size reduction and a 60% cell density promotion, while at lower temperatures, the stabilization led to greater cell size and reduced cell density.

Alightweight polymer composite with a broad bandwidth، tunable absorption frequency and multi-functionality is an ideal material for making a radar absorber. In general، composite microcellular foams have many potential applications due to their lightweight، high mechanical properties and monotonous cell structure. In this research، the effect of foaming method on the radar absorbing properties of composite radar absorbers was investigated. In the first step، a controllable، repeatable and high pressure/temperature operation foaming system by supercritical CO2 gas as foaming agent was designed and built. The composites based on poly (methyl methacrylate) (PMMA) and multiwall carbon nanotube (MWCNT) at different weight percentages were prepared by solvent-anti solvent coagulation method. The sample sheets with 3 mm thickness were molded using hot compression molding method. Then، the foaming process was performed and the cell morphology of the prepared foams was studied using scanning electron microscopy. Monotonous cell structure of the composite foams revealed a good dispersion of nanoparticles in the polymer matrix. The data of the reflected radar waves (before and after foaming) showed that the foaming reduced the reflection of the radar waves to less than 10 percent in all the samples. It is important to note that the absorption of radar waveswas increased with the foaming of neat PMMA. It was observed that the foaming of composites increased the threshold of absorption of radar waves from less than 1 wt% nanotube for the unfoamed samples to 1-3 wt% nanotube for the foamed samples.

Most of porous polymer materials are mainly prepared by solution phase inversion method. The de-mixing process can be initiated by diffusive solvent/non-solvent exchange of binary، ternary or multicomponent mixture. Recently، supercritical CO2 (ScCO2) is used as a non-solvent to prepare porous polymers. ScCO2 possesses excellent properties، such as environmental friendliness، liquid-like density and gas-like diffusivity in comparison with conventional liquid non-solvents. Porous polyurethane was prepared from polymer/N-dimethylformamide (DMF) solution using a supercritical fluid-phase inversion process in which carbon dioxide acted as the non-solvent. The effects of time delay، casting temperature and ratio of polyol/chain extender (POL/CE) on the membrane morphology and structure (size and distribution of cells and pores، thickness of dense layer and porosity) were studied. According to the results، a minimum time delay of 120 min was determined for formation of a suitable structure. The dense layer of 77. 6 μm thickness was decreased to 43. 1 μm by lowering casting temperature from 55°C to 35°C. The ratio of POL/CE influenced the thermodynamic and kinetic properties. A decrease in POL/CE ratio led to formation of smaller and uniform cells and increased porosity. On decreasing the ratio of POL/CE from 2 to 0. 25، the mean diameter of the cells at 6. 3 μm dropped to 3 μm.

Electromagnetic compatibility (EMC) and electromagnetic interference (EMI) have emerged as key issues with respect to commercial and military purposes in association with electromagnetic waves. The importance of protection against electromagnetic interference in wireless communication and electronic toll collection (ETC) systems has undoubtedly increased over the years. Generally، the electromagnetic absorption properties of material depend on their intrinsic electromagnetic properties such as conductivity، magnetic permeability and dielectric constant and also factors such as thickness and frequency. The effect of each parameter on the absorption performance is yet difficult to comprehend due to the complexity of electromagnetic waves propagation in different media. Addition of pure dielectric or magnetic material to a polymer matrix is a possible way to change electromagnetic properties of the materials. In this study nanocomposites of polystyrene/multi-walled carbon nanotubes were prepared using a solution method with three different homogenizer speeds for the purpose of nanotube dispersion and evaluation of the effect of nanotube dispersion on the electromagnetic wave absorption properties. The morphology of the nanocomposits was investigated by scanning electron microscopy (SEM). The capability and properties of electromagnetic wave absorption of nanocomposites were studied in the frequency range of 5 to 8 GHz using a vector network analyzer and finally the results of their absorption were compared with each other. It was found that by improving the dispersion of nanoparticles، both the amount and bandwidth of absorption increase. Moreover، by increasing the homogenizer speed up to 10000 rpm the maximum reflection loss was reported to occur at 8 GHz.

In this research, microcellular thermoplastic polyurethane foams are investigated as an absorbing material in the X-band (8.2-12.4GHz) frequency range by means of numerical analysis and experiment. In the frame of this work, we aim at establishing relationships between the foams morphology including cell size and air volume fraction and their radar absorbing properties.
We therefore first describe numerical method and modelling. Then numerical analysis of microcellular foams in various cell sizes and air volume fractions are explained. Then design basis and preparation of nanocomposite foams of various morphologies using supercritical carbon dioxide (scCO2) as physical foaming agent are presented. After measuring the S-Parameters of the samples by VNA, numerical and experimental results are compared and finally we establish structure/properties relationships that are essential for further optimizations of the materials for the radar absorbing applications.

This study was an effort to control foam structure with controlled shear rate in a slit die with addition of nanosilica in the extrusion process. We used general purpose polystyrene as the matrix، nanosilica as a nucleating agent and nitrogen gas as the blowing agent. For better dispersion of nanoparticles in the matrix، a combination of solution and melt methods were employed. The foams were produced in a twin screw extruder by studying the effects of shear rate on the slit die، size and weight percentage of nanosilica on foam density، cell density and average cell size. Scanning electron microscopicy (SEM) pictures were used to obtain the main characterizing parameters of the foams i. e.، cell density and cell size. The results showed that when the shear rate of 8 min-1 on the slit die was increased to 12 min-1، due to the breakage of the large cells into several smaller cells، there were observed 61% enhancement in cell density and 18% reduction in average cell size. When the size of nanosilica was decreased from 40 to 12 nm، the cell density was increased from 1. 24×108 to 1. 73×108 cells/cm3 and average cell size was decreased from 11. 5 to 9 micrometer. By changing the nanosilica content from 0. 1 to 4% (by weight)، it was observed that at 2% (by weight) of nanosilica، the maximum cell density was obtained. In conclusion it was observed that the nanocomposite was produced by 2% (by weight) of 12 nm silica nanoparticles at 12 min-1 shear rate shows the maximum cell density and minimum cell size.

nucleating agents are commonly used for controlling cell nucleation in the production of high-quality foam products. The surface properties of nanoparticles include geometrical characteristics, such as the particle shape, size, and size distribution and in this respect there are few reports on the effects of geometry of particles on foaming efficiency. This study is an effort to control polymeric foam structure with addition of silica nanoparticles in a batch process. In this research polystyrene (GPPS 1540) was used as a matrix, nanosilica as nucleating agent and supercritical carbon dioxide (CO2) as blowing agent. For better dispersionof nanoparticles in the matrix, a combination of solution and melt method was employed. In continuation, the effect of surface treatment of nanoparticle on polymer foam is investigated. First, silica nanoparticles were modified by reaction with vinyltriethoxysilane followed by characterization of the results. Then, the surface tension of polystyrene, silica and modified silica were determined by contact angle test in order to predict the dispersion of nanoparticles in polystyrene. Then, two kinds of polystyrene/silica (treated and non-treated) nanocomposites were prepared with different filler loadings by a combination of solution and melting methods. A high pressure/high-temperature vessel capable of instantaneous pressure release and controlled stabilization was used to prepare polystyrene-nanosilica nanocomposites foam. The effect of surface treatment of nanosilica particles on microcellular foaming was investigated by characterizing the cell density and cell size of the foamed productusing SEM. The results showed that surface-treated nanosilica has considerable effect on decreasing cell size and increasing the cell density.

Atheoretical hypothesis was put forward on the effect of nanoparticle size on cell nucleation in polystyrene (PS) matrix foam. At first the heterogeneous factor was calculated in different wetting angles from zero to 180º. Then، to verify this hypothesis a combination of the solution and melting methods was used to prepare PS-nanosilica nanocomposites at different concentrations and different sizes of nanosilica. In this regard a novel apparatus، including a high-pressure/ high-temperature vessel capable of instantaneous pressure release and stabilization control، was used to prepare a polystyrene-nanosilica nanocomposite foam in its melt state. To reach the foaming conditions، the vessel was pressurized with dry ice at the specified temperature. After reaching the foaming conditions، the sample was saturated with supercritical carbon dioxide (CO2) for 8 more hours. Then، the pressure was released with an on/off valve and the temperature was instantaneously dropped to below glass transition temperature. The scanning electron microscopy pictures were used to calculate the effect of nanosilica on the main foaming parameters including cell density and cell size. The results showed that a small amount of nanosilica has substantial effect on decreasing cell size and increasing the cell density. Moreover، the foam quality and structure was improved by addition of the nanoparticle.

A major problem in the chemical industries is the separation of waste material produced in the chemical processes. There has been a number of attempts to find a solution to this problem. One of these methods is sedimentation; however, the major drawback of this method is the low velocity of natural sedimentation. By adding particles with larger mass to this system, waste material could be absorbed on their surface and thus increasing the removing rate of the waste material. For this purpose, surface of massive material should be modified for adsorption of the waste material. However, sedimentation velocity is a function of hydrodynamic forces, particle-matrix interaction and particle-particle interaction so that change in surface properties can affect the sedimentation velocity. In this study, a model is developed to predict the effect of particle matrix interaction on sedimentation velocity. A method is also devised to measure sedimentation velocity and to obtain an estimate of particle size distribution. From this study, it is concluded that particle-matrix interaction could be used to increase sedimentation velocity.

Open-celled foams are capable to allow the passage of fluids through theirstructure, because of interconnections between the open cells or bubblesand therefore these structures can be used as a membrane and filter. In thiswork, we have studied the production of polystyrene open-celled microcellular foamby using CO2 as blowing agent. To achieve such structures, it is necessary to controlthe stages of growth in such a way that the cells would connect to each other throughthe pores without any coalescence. The required processing condition to achieveopen-celled structures is predictable by a model theory of opened-cell. This modelsuggests that at least a 130 bar saturation pressure and foaming time between 9 and58 s are required for this system. The temperature range has been selected for to beboth higher than polymer glass transition temperature and facilitating the foamingprocess. Experimental results in the batch foaming process has verified the modelquite well. The SEM and mercury porousimetry tests show the presence of poresbetween the cells with open-celled structure.

The effect of shear rate on dissolution of carbon dioxide in viscoelastic wheat flour matrix and cell density in a glass barrel twin screw extruder is investigated. It is found that by increasing the shear rate there will be a decrease in the required thermodynamic conditions and hence, it improves blowing agent dissolution and increases the cell density. Shear rate breaks up big bubbles and helps to better distribute the blowing agent in the matrix and hence it increases the cell density. Cell density decreases by dropping foam cooling rate. For a given cell density, a higher screw speed is needed, ifthe cooling rate is decreased.

The preparation and characterization of the vinyltriethoxysilane-modified silica nanoparticles were investigated. Also the surface tension of polystyrene, native (hydrophilic) silica and silane-modified (hydrophobic) silica were determined. Two kinds of polystyrene/silica (treated and non-treated) nanocomposites were prepared with different filler loadings by solution method. Their viscoelastic properties were studied by dynamic stress controlled rotary shear rheometer. Solid-like response of polystyrene / native silica nanocomposites were observed in the terminal zone. Solid inclusions increase the storage modulus more than the loss modulus, hence decrease the material damping. By increasing filler volume fraction, the particles tend to agglomerate and build clusters. The presence of clusters increases the viscosity, the moduli and the viscoelastic non-linearity of the composites.Treating the filler surface reduces its tendency to agglomerate as well as the adhesion between the particles and the polystyrene, leading to lower viscosity and interfacial slippage. Also the loss modulus peak is affected significantly by the particle surface area and its surface property in silica-filled polystyrene, which corresponds to its glass transition.