Silica Agglomerates Research Articles

Mechanical properties and morphological examination of isotactic Polypropylene/SiO2 nanocomposites containing PP-g-MA as compatibilizer

In the present study i-PP/SiO2 nanocomposites, containing 1, 2.5, 5, 7.5, 10 and 15 wt % SiO2 nanoparticles, were prepared by melt mixing on a twin-screw co-rotating extruder. Tensile and impact strength were found to increase and to be mainly affected by the content of silica nanoparticles. A maximum was observed, corresponding to the samples containing 2.5 wt% SiO2. The addition of PP-g-MA resulted in a further enhancement of mechanical properties due to the particle size reduction of silica agglomerates.

Thermal and scanning electron microscopy/energy-dispersive spectroscopy analysis of styrene-butadiene rubber-butadiene rubber/silicon dioxide and styrene-butadiene rubber-butadiene rubber/carbon black-silicon dioxide composites

Silica as a reinforcement filler for automotive tires is used to reduce the friction between precured treads and roads. This results in lower fuel consumption and reduced emissions of pollutant gases. In this work, the existing physical interactions between the filler and elastomer were analyzed through the extraction of the sol phase of styrene–butadiene rubber (SBR)–butadiene rubber (BR)/SiO2 composites. The extraction of the sol phase from samples filled with carbon black was also studied. The activation energy (Ea) was calculated from differential thermogravimetry curves obtained during pyrolysis analysis. For the SBR–BR blend, Ea was 315 kJ/mol. The values obtained for the composites containing 20 and 30 parts of silica per hundred parts of rubber were 231 and 197 kJ/mol, respectively. These results indicated an increasing filler–filler interaction, instead of filler–polymer interactions, with respect to the more charged composite. A microscopic analysis with energy-dispersive spectroscopy showed silica agglomerates and matched the decreasing Ea values for the SBR–BR/30SiO2 composite well. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2273–2279, 2005

Agglomerated non-porous silica nanoparticles as model carriers in polyethylene synthesis

Non-porous submicron silica particles (250 and 500 nm) with high monodispersity were agglomerated to form spherical agglomerates via spray drying. As a binder, 25 nm sized monodisperse silica spheres were selected from a variety of colloidal systems including Levasil-type and Aerosil-type silica nanoparticles. The use of such binders led to an increase of the specific surface area of the agglomerated carriers. All materials were characterised by nitrogen sorption, mercury intrusion and scanning electron microscopy. The silica agglomerates, with highly defined geometrical and pore structural parameters, were employed as model carriers in the heterogeneous polymerization of ethylene using a conventional methylaluminoxane (MAO)/η5-dicyclopentadienyl zirconiumdichloride metallocene catalyst system. The activity of these model carriers was evaluated, as was the influence of MAO impregnation on the carriers’ physico-chemical properties. The activity of the presented catalyst systems depended heavily on the percentage of binder. Due to skin formation, high contents of binder resulted in agglomerates with a smoother, more rigid surface. These carriers did not fragment during polymerization, and displayed low catalytic activity. Less binder gave more readily fragmented agglomerates displaying higher catalytic activities. Despite their small specific surface areas in comparison to commercially available polymerization carriers, e.g. SYLOPOL 948 from W.R. Grace, these model carriers possess similar activities, with the polymerization process depending more on the geometrical and structural aspects of the beads rather than on their specific surface areas.

Temperature-Programmed Desorption Study of the Selective Oxidation of Alcohols on Silica-Supported Vanadium Oxide

The partial oxidation of methanol and ethanol on silica-supported vanadium oxide catalysts was studied using temperature-programmed desorption (TPD), Raman spectroscopy, and diffuse reflectance infrared spectroscopy (DRIFTS). Methanol TPD results for V2O5/SiO2 samples as a function of vanadia loading in conjunction with X-ray diffraction data and Raman spectra indicated that dispersed vanadia on silica agglomerates into vanadia crystallites during a CH3OH TPD experiment. For ethanol-dosed samples, agglomeration of the dispersed vanadia was less severe, and it was possible to measure the activation energy for the dehydrogenation of adsorbed ethoxides to produce CH3CHO. Assuming a preexponential factor of 10(13) s(-1), the activation energy for this reaction was estimated to be 132 kJ/mol. The results of this study further demonstrate that there is a relatively weak interaction between vanadia and silica and suggest that adsorbed methoxide species help facilitate agglomeration of dispersed vanadia.

Infiltration of uncured elastomers into silica agglomerates

AbstractThe degree of impregnation of an agglomerate by the suspending matrix is an important parameter because it influences the agglomerate cohesion and thus has a direct effect on dispersion mechanisms. Penetration kinetics reported in the literature were only measured in Newtonian or not very elastic matrices, although one of the main applications is the dispersion of porous filler in elastomers. The aim of this article is to study the infiltration of porous silica agglomerates by uncured elastomers using optical microscopy. The application of the model of Bohin (infiltration of a Newtonian fluid into a porous sphere) is discussed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3292–3300, 2004

Continuous Mixing and Compounding of Polymer/Filler and Polymer/Polymer Mixtures with the Aid of Ultrasound

Abstract High power ultrasound is applied to break down silica agglomerates in EPDM/silica mixtures and to compatibilize plastic/rubber and rubber/rubber blends during extrusion process. Ultrasonically treated EPDM/silica mixtures without silane treatment of silica indicated significantly reduced sizes of silica agglomerates in comparison with those obtained by an internal mixer and lower than the EPDM/silica mixtures treated by the silane. The viscosity of the ultrasonically treated mixtures was found to be higher than that of the silane treated mixtures, while the extrudate swells of both mixtures were close to each other. Ultrasonically treated plastic/rubber and rubber/rubber blends indicated in-situ copolymer formation and compatibilization in the melt state leading to significant enhancement of the mechanical properties of the blends. An idea is put forward about the possibility of in-situ copolymer formation at the interfaces and their vicinities in the ultrasonically treated blends during extrusion. Data obtained by a solvent extraction indicating reduction in the amount of the extractable component, gel permeation chromatography indicating formation of the high molecular weight tail, atomic force microscopy indicating smooth interfacial region and scanning electron microscopy indicating stabilization of the phase morphology in the melt state of the ultrasonically treated blends point towards the occurrence of in-situ copolymer formation. The discovery creates a new field of the study of ultrasonic-assisted mixing of rubber with filler and compatibilization of immiscible polymer blends during extrusion.

Structure of silica in Equisetum arvense.

Silicified regions in the stem and leaf of the horsetail Equisetum arvensewere studied by scanning and transmission electron microscopy. The silica was present as a thin layer on the outer surface with variation in the size of this layer depending on the part investigated. There was a dense arrangement of silica spheres with some density fluctuations. A loose arrangement of silica particles with variation in their size was found beneath this dense arrangement suggesting the agglomeration of silica. An electron diffraction pattern showed the presence of amorphous silica, with the short range order being comparable to that of silica from other chemical sources. The medium range order shows the presence of silica with a high inner surface. SAXS measurements correlate with the particle size observed in TEM, and provide values for surface fractals. A new method of plasma ashing to remove the organics is also described.

Silica agglomeration and elastomer reinforcement: Influence of surface modifications

The properties of precipitated silica modified with hexadecylamine, dodecanol, 9-decen-1-ol and γ-mercaptopropyltrimethoxysilane have been characterised and related to elastomer reinforcement capability. Extensive modification with these agents decreased the specific surface area and the critical surface energy of silica, rendering filler with a reduced tendency to agglomerate. These changes in silica surface properties facilitated filler incorporation and dispersion in a tyre rubber compound, but diminished its reinforcement capacity as measured by bound rubber and stress-strain analysis. The data suggest that losses in surface energy support filler dispersion by reducing silica agglomeration, but reduce reinforcement capacity by weakening adhesion between filler and elastomer. However, optimisation of the extent of silica modification is feasible, given that the decline of silica agglomeration with respect to surface energy is more intense than the loss of elastomer reinforcement potential.

Influence of process parameters on the morphological evolution and fractal dimension of sol–gel colloidal silica particles

Colloidal silica particles were synthesized from tetraethoxysilane (TEOS), ammonium hydroxide (NH 4OH), ethanol (C 2H 5OH) and deionized water. Obtaining different structures was possible due to the variation of molar ratios of NH 4OH, C 2H 5OH and H 2O with respect to the precursor. The nature of the particles was evaluated using X-ray diffraction, energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). The morphology of the particles was described using electron microscopy (SEM). Pore structure was characterized by nitrogen gas sorption-desorption, showing colloidal silica particles with solid-pores surface areas varying from 0.48 m 2 g −1 up to 567 m 2 g −1; pore volume varying from 0.0013 up to 0.88 cm 3 g −1 and average pore radii varying between 1.4 and 36 nm. The fractal dimension of the silica particles and agglomerates varied from 2.0 to 2.12.

Silica surface modification using different aliphatic chain length silane coupling agents and their effects on silica agglomerate size and processability

Silica surface was treated with various aliphatic chain length silane coupling agents and compounded in EPDM using an internal mixer, and their agglomerate sizes, viscosity, and extrudate swell were investigated. The treated silica compounds showed smaller agglomerate size, lower viscosity, and lower swell reduction compared to untreated silica compound after equivalent mixing times. Short chain silane treated silica compound exhibited smallest agglomerate size. Silane acted as dispersing agents and processing aids in silica/EPDM compounds.

An improved procedure for packing capillaries for capillary electrochromatography

This paper introduces a novel improvement to high pressure slurry packing of CEC capillaries. Slurry homogeneity is maintained by constantly stirring in a ‘balanced density’ solvent system in a vessel with a configuration that prevents silica agglomerates from blocking the capillary inlet. Up to nine, one metre length capillaries may be packed simultaneously and repeatably in under 3 hours. All capillaries produced peak efficiencies greater than 100,000 plates/metre with 5 μm Spherisorb ODS1 and showed capillary to capillary variability of less than 10% in terms of peak symmetry and peak efficiency. The data has been presented in part, at the 23rd International Symposium in Chromatography 2000.

Characterisation of stoichiometric sol–gel mullite by fourier transform infrared spectroscopy

Fourier Transform Infrared Spectroscopic investigation has been carried out to study the structural changes and coordination of Al and Si in the mullite system as a function of temperature (60–1400°C). The FTIR spectral patterns of the gel and the gel calcined at 200°C show bands corresponding to Al–O–Al and Si–O–Si vibrations. At higher temperatures hydroxyl groups and organics present have been removed. The absence of fine structure in the region 400–900 cm−1 in the temperature range 400–1000°C is interpreted as due to the existence of nanostructured alumina, silica and alumina–silica agglomeration in the amorphous state. Analysis of the spectra indicates the presence of both octahedral and tetrahedral coordination at 1000°C, the temperature around which spinel is formed. The shifting of the asymmetric stretching frequencies of the Si–O–Si and Al–O–Si networks to higher wave numbers indicate the formation of mullite at 1200°C. Crystalline nature of the product is indicated by the sharpening of spectral peaks. The spectra also indicates the completion of mullite formation at 1250°C.

Synthesis of iron zircon coral by coprecipitation routes

An iron-doped (15 mol-% of Fe2O3) zircon ceramic pigment has been prepared using binary (ZrO2-Fe2O3 and SiO2-Fe2O3) and ternary (ZrO2-Fe2O3-SiO2) colloidal gels or coprecipitates as precursors. The obtained raw powders, precalcines, and fired pigments have been characterized by XRD, thermal analysis (TGA/DTA) and SEM/EDX to analyze the effect of binary interactions (occlusion and adsorption phenomena) on the synthesis of the iron-zircon coral as well as on the coloring yield. The use of a binary raw coprecipitate as precursor prepared with colloidal silica and ferrous sulphate leads to a higher efficiency in the hematite occlusion, since a more intense coral hue (L* = 59.6, a* = 29.3 and b* = 25.8 at 950°C) is obtained, similar to an optimal ceramic reference. The protection or occlusion of α-Fe2O3 in amorphous silica agglomerates of high specific surface appears to be a more effective reaction intermediate than the observed adsorption of micronic α-Fe2O3 particles on tetragonal zirconia monoliths. The necessary transformation of tetragonal zirconia into its monoclinic form prior to zircon formation, and the higher α-Fe2O3 segregation from the coprecipitate obtained with Zr and Fe precursors, seem to lower the efficiency of the hematite coarsening-occlusion process involved in the coral pigment formation.

Collision-induced dispersion of agglomerate suspensions in a shear flow

Agglomerates suspended in a polymer fluid have been known to disperse in a flow through two mechanisms-rupture and erosion. Using silica agglomerates, it is shown here that a third mechanism can occur, i.e., detachment of fragments due to agglomerate collision. This mechanism requires a much lower overall stress than erosion (and rupture, which occurs at even larger stresses than erosion). The fragment concentration produced by collision at a given time is proportional to the square of the applied shear rate.

Particle Sizes of Fumed Silica

Fumed silica is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame. Surface areas range from 50 up to 400 m2/g. Using particle sizing techniques, fumed silica shows micro-sized particles leading to surface areas markedly lower than expected. Fumed silica appears as a fluffy solid with bulk densities down to 0.03 g/cm3, being invariant over the wide range of surface areas. Attempts to relate the variation of the surface area directly to the performance of fumed silica in technical applications, such as its thickening efficiency in fluids, mainly fail and remain ambiguous. The aim of this work was to investigate the particle sizes and structures of fumed silica aggregates and agglomerates, using different particle dispersion and sizing techniques.

FTIR Analysis of Silica-Filled Natural Rubber

Abstract The interactions of silica filler in zinc activated, sulfur vulcanized cis-1,4-polyisoprene (IR) were characterized using Fourier Transform Infrared (FTIR)-Attenuated Total Reflectance (ATR) spectroscopy. The variables examined were increased levels of silica and quality of mixing of the batch. The increased silica level resulted in band broadening and a frequency shift to lower wavenumbers of the silica absorbance region (1250-1000 cm−1) due to a combination of physical and chemical adsorption of the rubber and parts of cure system on the silica surface. The peaks that appeared near 1040 and 1017 cm−1 were attributed to physical interactions between silica and the natural rubber. However some chemical bonding of the IR was present as a result of a free radical reaction that takes place during the milling process. Inefficient mixing had an affect comparable to adding less filler, according to the results above. That is, there was less “effective” filler in the poorly mixed samples because poor dispersion promotes agglomeration of the polar silica in the nonpolar hydrocarbon rubber.

Penetration of Silicone Polymers into Silica Agglomerates and Its Influence on Dispersion Mechanism

Abstract The dispersion of silica agglomerates suspended in silicone fluids undergoing simple shear flows has been studied. Two different breakup mechanisms, denoted “rupture” and “erosion” have been observed. These two breakup modes produce significantly different fragment size distributions. Penetration of the suspending fluid into the silica agglomerate was found to affect the mode and the conditions necessary for dispersion of the agglomerate. Silica agglomerates totally penetrated by silicone fluids resist dispersion much better than unpenetrated agglomerates. The kinetics of matrix penetration was modeled on the basis of capillary forces driving the penetration and viscous effects resisting it. Agreement of this model with experimental observations is excellent.

Energy—size reduction laws for ultrasonic fragmentation

The energy requirement is a key criterion for the selection and use of a grinding process. Ultrasonic dispersion is extensively used to disperse submicron agglomerated powders in liquid suspensions. Suspensions of silica agglomerates were ground with solids concentration up to 50% by weight. The fragmentation or grinding rate is inversely proportional to suspension volume. Starting from a semiempirical expression that relates fragmentation rate to particle size, suspension volume and ultrasonic power, energy consumption laws for both eroding and non-eroding powders are developed. Experimental results supporting the energy consumption laws are given. Lower power input for ultrasonication favors efficient energy use. For eroding powders (e.g. silica, zirconia) the energy expenditure per unit powder mass (specific energy) by ultrasonic grinding is lower than that of conventional grinding techniques. In contrast, it is slightly higher than ball milling for non-eroding powders (e.g. titania).

Dynamic mechanical properties of silica‐filled ethylene vinyl acetate rubber

AbstractDynamic mechanical studies of ethylene vinyl acetate rubber indicated the presence of different transitions: the α‐transition or glass‐rubber transition temperature (Tg) at −17°C, the γ‐transition around −60 to −70°C observed as a shoulder, and the γ‐transition in the temperature region of −127 to −135°C, apart from a high‐temperature transition (T1,1) at +69°C. The α‐transition temperature did not shift its position with incorporation of silica filler, but there was gradual sequential lowering of tan δ peak values at Tg with increase in filler loading. Such sequential dependence of tan δ values on filler incorporation was not observed in the cases of β‐ and γ‐transitions. The T1,1 transition was found to be less prominent in the presence of filler, particularly at high concentration. Treatment of the filler with a silane coupling agent caused a shift in the α‐transition temperature to higher temperature by about 4°C. Studies on dynamic mechanical properties under isothermal conditions have shown that the coupling agent caused a breakdown of silica agglomerates during mixing. An increase in frequency caused a shift of tan δ peak positions in the α‐ and γ‐transition regions toward higher temperature. Although frequency did not have a significant effect on the tan δ value at the α‐transition, increase in frequency caused sequential lowering of tan δ value at the α‐transition. increase in frequency caused sequential lowering of tan δ value at the γ‐transition. In the high‐temperature region, the T1,1 peak exhibited by the gum compound gradully disappeared with increasing frequency. © 1993 John Wiley & Sons, Inc.

Hydrodynamic behaviour of aerogel powders in high-velocity fluidized beds

The behaviour of silica aeroget powder and iron oxide—silica aerogel powder in fluidized beds has been observed at gas velocities between 0.1 and 1.6 m/s to determine whether aerogels can be treated in turbulent and circulating fluidized bed (CFB) contactors. Pure silica aerogel particles agglomerate at these velocities to create larger particles whose equivalent diameter is a strong function of the gas velocity at which they are formed. Although they retain the high specific surface area of the original powder, the agglomerates have a low permeability. It was never possible to smoothly circulate pure silica aerogel in a CFB unit with an L-valve, because the fine material could not be returned to the base of the riser, and large agglomerates remained suspended in the riser. However, by adding a granular diluent, it was possible both to suppress much of the agglomeration and to form a mixture ideal for CFB contacting. Iron oxide—silica agglomerates were homogeneous in size and insensitive to the gas velocity. Tests in a small CFB unit with a V-valve showed that iron oxide—silica aerogel particles could be circulated smoothly and steadily, because fluidized aerogel particles in the V-valve leg reorganized themselves into agglomerates which behaved like Geldart's Group A or B powders.