Polystyrene Powder Research Articles

Preparation of inorganic/polymeric composite microspheres by direct suspension polymerization

Spherical polystyrene powders coated uniformly byγ-Fe2O3 or RuO2 were obtained by means of a direct suspension polymerization. Starting reactant mixture was a complex disperse system comprising a fine powdered material, droplets of styrene monomer containing lauroyl peroxide as an initiator; and an aqueous medium. By varying the ratio between monomer and water, amount of powders as well as of the initiator, factors dominating particle size, volume fraction-of fine powders adhered and the degree of polymerization were elucidated

Dielectric and pyroelectric properties of TGS-polystyrene composites

Abstract A composite of triglycine sulphate (TGS) and polystyrene has been fabricated in a multilayer pattern with 2–2 connectivity made up of alternating layers of polystyrene and TGS powder. Samples with TGS powders of varying particle size, ranging from less than 75 μm to more than 400 μm were prepared in the form of thin sheets (2.0 cm × 1.5 cm × 0.15 cm). Samples with same weight percentage (70 to 75%) of TGS, but different particle size, were chosen for dielectric and pyroelectric studies in the ferro- and paraelectric phases. The dielectric constant and loss have been measured in the frequency range of 0.1 to 100 KHz and the pyroelectric current measurements were made by the Byer and Roundy direct method. The magnitude of dielectric constant (ϵ′) and pyroelectric coefficient Py show an increase with increasing particle size and the room temperature values for the biggest particle size are [ϵ′ = 8.3] and Py = 0.6 × 10−10 C cm−2 K−1. Anomalies in ϵ′ and pyroelectric coefficient, indicating the existence of ferroelectricity were observed even for the composites with the lowest particle size of TGS (< 75 μm). A dielectric dispersion in the frequency range 0.1 to 100 KHz and the particle size dependence of pyroelectric coefficient and dielectric constant can be accounted for by assuming non-ferroelectric surface layers at the boundaries of TGS crystallites. The responsivity of TGS-Polystyrene composites for infrared detection has been compared with those of single crystals, hot pressed TGS powders and composites employing other ferroelectrics. The decrease in pyroelectric coefficient with particle size is compensated to some extent by the decrease in dielectric constant, such that responsivity of these composites does not show any marked dependence on particle size.

Cryogenic crushing of polystyrene

AbstractAtactic and isotactic polystyrene powders were fractured with a ball mill at −196°C. The molecular weight distributions at various milling times measured by GPC show an initial asymmetric broadening and then a gradual returning to the original narrow bell‐shaped distribution. Analysis of molecular weight distributions suggests a preferential chain scission near the chain ends. X‐ray diffraction spectra of isotactic polystyrene samples show that after 24 hr of milling, the semicrystalline polystyrene was completely transformed into an amorphous polymer. DSC thermogram of the milled isotactic polystyrene reveals several crystallization peaks preceding melting when the samples were heated.

On the rheology of packed polymer powders

AbstractThe rheology of polystyrene powder beds was examined by two kinds of experiments. From stress relaxation measurements during powder compaction, it was found that relaxation at the shorter times could be attributed to sliding between particles. This is followed by a relaxation attributed to the deformation of the polystyrene particles themselves. From dynamic mechanical measurements, an increase was found in the value of vibrational absorption coefficient (Q−1 value) at a temperature below that of the main relaxation dispersion of bulk polystyrene, attributed to the slipping around tight particle junctions. These phenomena appear characteristic of powder‐like materials.

Induction and measurement of glassy‐state relaxations by vapor sorption techniques

AbstractRecent gravimetric studies of the sorption of organic vapors by poly(vinyl chloride) and polystyrene powders have demonstrated several features which promise to be generally useful in studying the structure and properties of the glassy state. The uptake of vapor can be significantly altered by prior thermal or vapor treatment of the polymer, apparently reflecting changes in the microvoid content or free volume of the polymer. Fickian sorption in sufficiently fine powders proceeds to equilibrium in a few minutes. Upon exposure of a polymer powder to an appreciable pressure of vapor, both a rapid Fickian sorption and a slower, relaxation‐controlled sorption are observed. Superposition of these processes leads to widely varied sorption kinetics; a model comprising Fickian diffusion and first‐order relaxation terms accurately describes the data and allows estimation of equilibrium and rate constants for both processes. After prolonged exposure, removal of a swelling vapor induces a slow reconsolidation of the polymer structure; this deswelling relaxation can be monitored by the decreasing amounts of vapor sorbed in repeated brief exposures to low vapor pressures, and can also be described by a first‐order relaxation model. In this regard, the penetrant vapor serves as a molecular probe, monitoring glassy‐state relaxation occurring in the absence of penetrant. The same, presumably true equilibrium is ultimately reached both by swelling from a low free‐volume state and by consolidation from a preswollen state of high free volume. The rates of both swelling and consolidation relaxations appear to be retarded by the presence of low concentrations of vapor in the polymer, suggesting that vapor molecules may preempt some of the free volume required for relaxation.

Oxidative Degradation of Polymers. VI. Photooxidation of Polystyrene Powder and Some Model Compounds

Abstract The rates and products of photooxidation of polystyrene powder were studied at 70 °C under irradiation of a 30W low pressure mercury lamp. It was found that the rate of oxidation is remarkably high but the amount of oxygen incorporated into the polystyrene chain is very small. About a half of the absorbed oxygen appeared as carbon dioxide, independent of the conversion, irradiation time, oxygen pressure, and molecular weight. A trace amount of phenol was detected by GLC. The isotachophoretic analysis showed the formation of formic acid, oxalic acid, and glyoxylic acid. It was found that the mechanism of photooxidation is dependent on the state of the substrate, i.e., crystalline solid, melt(viscous liquid) or solution. It is concluded that the polystyrene powder is photooxidized from its surface and aromatic ring of polystyrene is cleaved to give carboxylic acids. It was suggested that ozone played an important role in the photooxidation of polystyrene. The conclusions are supported by the results obtained in the photooxidation of some model compouds such as 1,2-diphenylethane, hexamethylbenzene, azobenzene, and phenylazotriphenylmethane.

Rheoiogical studies of compaction for polystyrene powder

AbstractPolystyrene powder was compacted in a cylindrical die at constant compaction speed. Compaction was found to be influenced by test temperature, compaction speed and void fraction of the powder bed. The stress reduction behavior of an axial stress σ1 and a radial stress σ2 was observed at constant void fraction after compaction. The stress reduction thus obtained was interpretable by adopting an octahedral normal stress σoct and an octahedral shear stress τoct which were converted from σ1 and σ2. This behavior was attributable to the deformation of polystyrene around contact points and to a sliding mechanism between particles. It was ascertained that the relation between a shear stress τ and a normal stress σ obtained on compaction differed from that obtained in shear by a mechanism involving movement between polystyrene particles.

Effect of Nitric Acid-Treatment on γ-Loss Band in Linear Polyethylene

In order to obtain more information on molecular mechanisms for three relaxations in the γ-loss band in linear polyethylene, the effect of the treatment with HNO3 at 80°C on the magnitude of the loss tangent associated with these relaxations is investigated. Values of the loss tangent for polyethylene are calculated from those for a composite solid made of polystyrene and polyethylene powder treated variously. For this purpose an extension of the applicability of the empirical formula proposed by Gray and McCrum is examined. It is evident that the γ1 and the γ2-relaxations decrease and the γ3-relaxation increases in magnitude with increase in the treatment time. From these results the γ3-relaxation is attributed to the motion of cilia which is increased by the treatment, while the other two relaxations are attributed to the motion of folds or other interlamellar amorphous parts.

Cold compaction molding and sintering of polystyrene

AbstractIt is the objective of this paper to demonstrate the applicability of cold compaction molding followed by a sintering treatment to the processing of polystyrene powders. The influence of pressure, compaction speed, and peak pressure dwell time on the green (as compacted) density and the green tensile strength, as well as the effect of sintering on the tensile strength and dimensional change, were evaluated. The resulting data indicate that room temperature compaction alone is insufficient to provide adequate tensile strength for the compacts. Sintering the green compacts at temperatures of 150 to 173°C markedly improves the tensile strength while simultaneously causing a thickness change in the compacts. This thickness change results from gas evolution, pore shrinkage, and viscoelastic recovery of the residual stresses induced by pressure. For compacts of 0.225 in. thickness, an optimum sintering treatment of 173°C for 30 mins is recommended to provide a tensile strength of 4,000 psi and a thickness change of less than + 7 percent. Coining (repressing) the green compacts does not appreciably affect the sintered strength. However, a finer particle size improves the sintered properties. A review of the literature on the flow behavior of polystyrene suggests that a non‐Newtonian viscous flow mechanism is followed by a Newtonian one as sintering progresses.

An Investigation of Electrostatic Charging Effects in High-Speed Gas-Solids Pipe Flows

A comparison is made between the pressure distributions in identical brass and nylon pipe-lines conveying polystyrene powder, showing that electrostatic charging of the solid particles has a negligible effect on highspeed, gas-solids pipe flow. A theory is presented for the prediction of the electrostatic charging rate of the solid particles. It is predicted and confirmed experimentally that the current to earth from electrically isolated sections of the brass pipe and from aluminium foil wrapped on a length of the nylon pipe varies linearly with the solids loading.

X-Ray investigations of flowing powders

X-Ray radiographs of polystyrene powder have been obtained in the course of an investigation into their mechanism of flow from hoppers. Whenever a radiograph was taken of a powder after flow out of an aperture, distinct and discontinuous internal surfaces of low powder density were seen. It is shown that the properties of these surfaces are consistent with their being along the slip directions predicted for failure of a plastic material. A cine film is available in which the visual flow that can be seen through the sides of the hoppers is compared with cine X-ray film in which the discontinuous lines can be seen forming and moving during the flow and with their stationary pattern when flow is stopped. The significance of these experimental observations is discussed, with special reference to what can be deduced from them about the stress fields.

Effects of Compacting Pressure and Temperature on the Structure of the Polystyrene Powder Compacts

In order to elucidate the hot-pressing process of granular materials, studies were made of the effects of compacting pressure and temperature on the structure of polystyrene powder compact. The kinetic studies showed that within the temperature and pressure range studied, the relation between the percentage of shrinkage S of the powder compact and the compacting time t was expressed by an empirical equation, S=Atk (t>30sec), in which k was an apparent rate of shrinkage. At a given compacting pressure the temperature dependence of the apparent rate of shrinkage k is characterized by a sharp maximum at a critical temperature Ts. It was found that the critical temperature did not depend on the shape and size of powder particles as was characteristic of polystyrene. This Ts decreased gradually with increasing compacting pressure. The powder compacts had different pore structures depending on the compacting conditions. Thus, for those prepared below Ts pore size and its distribution remained almost unchanged with compacting time, whereas for those above Ts a remarkable change was observed in both of them.As a conclusive summary of these results and discussion, two different mechanisms of hot-pressing of polystyrene powder compacts were suggested, i.e., local deformation of the powder at the contact points below Ts and the overall deformation caused by the softening of polystyrene above Ts. The results of microscopic observation are in favor of the suggested mechanisms.

On the polymerization of formaldehyde in liquid state. I. Gamma ray‐initiated polymerization of the monomer in solution

AbstractThis paper presents a more detailed discussion on the kinetics of formaldehyde polymerization under gamma‐irradiation than the previous paper2. Polymerization took place in toluene and ethyl bromide and within the range of −50°∼−80°C.The overall reaction is approximately third‐order with respect to the monomer. The average molecular weight of the polyoxymethylene obtained is little dependent on the initial concentration of the monomer and on dose rate. The reaction is approximately first order with respect to dose rate. It is believed from the facts obtained here that the polymerization, in which the transfer process of growing chain to monomer is mainly responsible for governing the molecular weight, should proceed predominantly by an ionic mechanism.A study of the influence of various additives upon the polymerization shows the following results. These additives are classified in four groups with respect to their qualities, the first group having influence only upon the polymerization rate (e.g., diphenylpicrylhydrazyl), the second only upon the molecular weight (water), the third upon both of these factors (methyl formate, methyl acetate, ethyl acetoacetate, polystyrene powder, and tetrahydrofuran), and the last having no influence upon both of these factors (oxygen, carbon monoxide, nitrobenzene, and naphthalene).The apparent activation energy is estimated to be 4.15 kcal/mole from rate of polymerization, −3.50 kcal./mole from average degree of polymerization.

Pore Structure of Plastic Powder Compacts

The pore structure of hot-pressed polystyrene and polyvinyl chloride powder compacts prepared at 40-100°Cand 1-8t/cm2 was studied by means of mercury porosimeter. Most polystyrene compacts have two peaks in the open pore size distribution curve, one at 0.01μ and the other at 1-5μ inradius. It has been found that the first peak corresponds to the pore on the surface of starting powder particle and the second to interparticle voids. The latter has been found to shift to smaller radius with increasing compacting pressure and/or temperature. The effects of compacting conditions on open and total pore volumes were studied. And it has been shown that the compacting pressure at which the open pore volume remains unchanged with further increase of pressure decreases as compacting temperature is raised.The increase in the open pore volume was observed on heating the polyvinyl chloride powder compacts prepared at room temperature. This might be explained by irreversible displacement of particles due to the expansion or stretching of particles and other factors. Different features both in the pore structure and its dependence on compacting conditons were observed between the poly-styrene and polyvinyl chloride powder compacts. This would be ascribed to the differences in their softening characteristics and powder parameters.On the basis of the experimental results and discussion, the mechanism of pore volume variation has been presented, and it is suggested that the deformation caused by softening and compression of powder particles is one of the most important factors to be considered in plastic powder compact.