The effect of opacifier properties on thermal conductivity of vacuum insulation panel with fumed silica

Journal of Thermal Analysis and Calorimetry - The reduction of radiation heat transfer in VIPs is an important issue. In VIPs, the radiation heat transfer is reduced by the use of opacifier....     

Effects of incorporation of modified silica nanoparticles on the mechanical and thermal properties of PMMA

Silica nanoparticles of various sizes have been incorporated by melt compounding in a poly(methyl methacrylate) (PMMA) matrix to enhance its thermal and mechanical properties. In order to improve nanoparticles dispersion, PMMA grafted particles have been prepared by atom transfer radical polymerization (ATRP) from well-defined silica nanoparticles. This strategy was expected to ensure compatibility between both components of the PMMA nanocomposites. TEM analysis have been performed to evaluate the nanosilica dispersion whereas modified and non-modified silica/PMMA nanocomposites thermal stability and mechanical properties have been investigated by both thermogravimetric and dynamical mechanical analysis.     

Preparation and mechanical properties of bacterial cellulose nanocomposites loaded with silica nanoparticles

Bacterial cellulose (BC), which is produced by Gluconacetobacter xylinus (Ga. xylinus) in culture, is made up of a three-dimensional network of ribbon-shaped bundles of cellulose microfibrils. In the current studies, we used two processes to prepare nanocomposites of BC filled with silica particles. In Process I, Ga. xylinus was incubated in medium containing silica sol Snowtex 0 (ST 0, pH 2–4) or Snowtex 20 (ST 20, pH 9.5–10.0). The elastic modulus at 20 °C was improved by keeping the amount of silica in the nanocomposites below 4% when ST 20 was used and below 8.7% when ST 0 was used. This process allowed incorporation of 50% silica in BC. Inclusion of higher amounts of silica reduced the modulus at 20 °C and the strength of the nanocomposites below that of BC. X-ray diffraction measurements revealed that the silica particles disturb the formation of ribbon-shaped fibrils and affect the preferential orientation of the ( ) plane. We also produced BC-silica nanocomposites by Process II, wherein the BC hydrogel was immersed in different concentrations of silica sols, allowing silica particles to diffuse into the BC hydrogel and lodge in the spaces between the ribbon-shaped fibrils. This method increased the modulus at 20°C and the strength compared to the BC matrix, but it was difficult to load the BC with more than 10% silica in this way.     

Surface hydroxylation and silane grafting on fumed and thermal silica

The optimization of the surface functionalization of flat thermal silicon oxide by silanes was investigated. The difficulties are the low density of silanols at the surface of thermal silica, the lack of precise knowledge of the actual surface chemistry of thermal silica and of its hydroxylation, and the limited number of possible chemical analyses at flat surfaces of small area. This steered our study toward a comparative investigation of the hydroxylation and silane grafting of thermal silica and the well-known fumed silica. The silane grafting density for fumed silica that had undergone thermal treatments of dehydroxylation was related to the surface density of silanols. The surface density of silane on the flat thermal silica as measured by FTIR-ATR spectroscopy was 1.4 micromol/m2, similar to that of fumed silica dehydroxylated at 1000 degrees C. This moderate value was related to the low silanol density present on such silica surfaces. Several rehydroxylation treatments that proved their efficiency on dehydroxylated fumed silica did not lead to any noticeable improvement on thermal silicon dioxide.     

Synergistic effect between POSS and fumed silica on thermal stabilities and mechanical properties of room temperature vulcanized (RTV) silicone rubbers

In this paper, both divinyl-hexa[(trimethoxysilyl)ethyl]-POSS (DVPS) and fumed silica were firstly introduced into polydimethylsiloxane (PDMS) system using as the cross-linker and the reinforcing filler respectively. And a series of novel RTV silicone rubbers synergistically enhanced by DVPS and fumed silica were prepared. The cross-linked networks in the novel RTV silicone rubbers have been studied by attenuated total reflection infrared spectroscopy, and the dispersions of POSS and fumed silica in these novel RTV silicone rubbers have been observed by means of scanning electron microscope (SEM). And thermal stabilities, thermo-oxidative stabilities and mechanical properties of these novel RTV silicone rubbers were studied by means of thermal gravimetric analysis and universal tensile testing machine, respectively. From the obtained results, it was found that synergistic effect between POSS-rich areas and fumed silica on thermal stability and mechanical property of RTV silicone rubber indeed existed. And the experimental results also exhibited that the thermal stabilities and mechanical properties of the novel RTV silicone rubbers were far better than those of the reference materials (DVPR and MTFR). The striking enhancements in thermal properties and improvements on mechanical properties of novel RTV silicone rubbers were likely attributed to the synergistic effect between POSS-rich domains and fumed silica. Meanwhile, it was found that the mechanical properties of RTV silicone rubbers prepared with a given amount of POSS cross-linker were enhanced with the increment of the loading amount of fumed silica.     

Dispersion stabilities and rheological properties of fumed silica suspensions

Herein, we have reviewed fumed silica suspensions in dispersing fluids, polymer melts, and polymer solutions, focusing on their dispersion stability and rheological properties as a function of the surface character of fumed silica powders and the silica volume fraction, ?. Hydrophilic fumed silica powders are well dispersed at ? < 0.01 in polar dispersing fluids or polar polymer melts, and their phase states change from sol to gel with increasing ?. Such changes should also be strongly related to the rheological responses of the hydrophilic fumed silica suspensions, which change from Newtonian flow behavior to gel-like elasticity with increasing ?. On the other hand, hydrophobic fumed silica powders are stabilized in both polar and nonpolar dispersing fluids, depending on the interactions between the surface hydrophobic moieties and the dispersing fluids, in addition to those between the residual surface silanol groups and dispersing fluid, except for the particle–particle interactions. Moreover, the effects of the adsorption and desorption of polymers, as well as of non-adsorbing polymers on the dispersion stability and rheological behavior of fumed silica suspensions are discussed, by taking account of their optical microscopic observation and SANS curves.     

The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA

Silica-PMMA nanocomposites with different silica quantities were prepared by a melt compounding method. The effect of silica amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics of PMMA was investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), Fourier-transform infrared spectroscopy (FTIR), ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (¹³C{¹H} CP-MAS NMR) and measures of proton spin-lattice relaxation time in the rotating frame (T_1ρ(H)), in the laboratory frame (T₁(H)) and cross-polarization times (T_CH). Results showed that silica nanoparticles are well dispersed in the polymeric matrix whose structure remains amorphous. The degradation of the polymer occurs at higher temperature in the presence of silica because of the interaction between the two components.     

Effect of silica content on thermal stability of fumed silica/epoxy composites

Composites of a fumed silica industrial residue and an epoxy resin were prepared and their thermal stability and thermal degradation behaviour were studied by TGA in air. Classical thermal stability parameters, based on the initial decomposition temperature (IDT), temperature of maximum rate of mass loss (T_max) and integral procedure decomposition temperature (IPDT) were calculated before and after subtraction of the filler mass from the TGA curves. Without filler mass subtraction, the thermal stability of the epoxy resin seems to be improved and the mass loss rate was reduced by the addition of fumed silica. Nevertheless, after subtraction of the filler mass, the thermal degradation behaviour of the resin was only slightly affected by the silica content. A possible negative effect of the silica content on the cure was also found.     

Morphological hysteresis in immiscible PIB/PDMS blends filled with fumed silica nanoparticles

The morphological hysteresis behavior of immiscible polymer blend reflects the dependence of their steady-state morphology on the shear protocol applied. In this work, the influences of hydrophobic and hydrophilic fumed silica nanoparticles on the morphology hysteresis behavior of immiscible polyisobutylene (PIB)/polydimethylsiloxane (PDMS) (10/90) blends under simple shear flow were investigated by using optical shear technique. Compared with particle-free blend, the morphology hysteresis zone of filled blends was found to be expanded by the addition of hydrophobic or hydrophilic fumed silica nanoparticles. It was found that the expansion of the morphology hysteresis zone in hydrophobic nanoparticle-filled blend stemmed from the suppression of droplet coalescence. However, the expansion in the morphological hysteresis zone for hydrophilic nanoparticle-filled blend, which was less noticeable, might originate from the more difficult breakup of PIB droplets upon the addition of nanoparticles.     

Crystallization and thermal properties of PLLA comb polymer

Poly(L ‐lactide) (PLLA) on poly(2‐hydroxyethyl methacrylate) (PHEMA) backbone was prepared by a combination of atom transfer radical polymerization (ATRP) and ring‐opening polymerization (ROP). The structure of the comb polymer was analyzed by wide angle X‐ray diffraction (WAXD), small angle X‐ray scattering (SAXS), and differential scanning calorimetry (DSC). WAXD result indicates that the comb polymer has α crystalline modification with a 10₃ helical conformation. Lamellar parameters of the crystalline structure were obtained by one‐dimension correlation function (1DCF) calculated from SAXS results. The calculations show that the thickness of crystalline layer is controlled by annealing temperature and comb structure. DSC was applied to study kinetics of the crystallization and melting behavior. Two melting peaks on melting curves of the comb polymer at different crystallization temperature were detected, and the peak at higher temperature is attributed to the melt‐recrystallization. The equilibrium melting temperature is found to be influenced by the comb structure. In this article the effects of the comb structure on Avrami exponent, equilibrium melting point and melting peak of the comb polymer were discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 589–598, 2008     

Characterization and thermal properties of Ni-exchanged montmorillonite with benzimidazole

Thermal analysis (TG, DTG), powder diffraction analysis (XRD) and infrared (IR) spectra were used to study of composition and release of benzimidazole from Ni(II)-exchanged montmorillonite under heating. Diffraction analysis indicated that benzimidazole molecules are intercalated into the interlayer space of montmorillonite. IR spectra and the analytical characteristics have shown that different type of interactions of benzimidazole is connected with different reaction conditions (acid or neutral solution). The release of benzimidazole from Ni(II)-montmorillonite under heating from studied samples proceeds in three distinct steps. The first step can be assigned to the release of water molecules while the last (third) one corresponds to the lattice dehydroxylation. The second step can be assigned to release of chemically bonded benzimidazole.     

Enhanced thermal and mechanical properties and biostability of polyurethane containing silver nanoparticles

A polyether-type polyurethane (PU) containing silver (Ag) nanoparticles (4-7 nm, 1.51 × 10⁻³-1.13 × 10⁻² wt-%) was prepared by mixing the waterborne PU with the nanoparticle suspension, casting and drying at 60 °C. The Ag nanoparticles were found to be well dispersed in PU. A significant increase in the thermal stability and mechanical properties of the polymer was demonstrated in the nanocomposite PU films, which was believed a result of induced crystallization in the presence of Ag nanoparticles. The biostability was tested in a rat subcutaneous model. After 19 days of implantation, the PU containing Ag showed enhanced biostability and lowered foreign body reaction. The effect of Ag on the stability of the PU polymer was even more remarkable over a wider range of particle contents than that of the gold nanoparticles previously studied.     

Properties of biodegradable poly(butylene carbonate) (PBC) composites with fumed silica nanoparticles

Biodegradable poly(butylene carbonate)/fumed silica (PBC/SiO₂) nanocomposites were prepared by melt compounding. The PBC/SiO₂ nanocomposites exhibited a good dispersion of aggregates of SiO₂ in the PBC matrix, and an improvement in mechanical properties. Nanoparticles affect, also, the thermal properties of PBC and especially the crystallization rate, which in all nanocomposites is faster than that of pure PBC. Due to ongoing crystallization and the crystal perfection during heating process, the melting peak of PBC shifted to higher temperature when heating from amorphous state with decreasing heating rate. With increasing cooling rate, the non-isothermal crystallization exotherms became wider and shifted to lower temperature. At a given cooling rate, the crystallization peak temperature of neat PBC was lower than that of its nanocomposite. Non-isothermal crystallization kinetic procedure, the method of Ozawa, was applied to the first deconvoluted DSC peak only by processing the data related to DSC peak. The average value of Ozawa exponent m of pure PBC is 3.04, while the one of its nanocomposite is about 2.98. Moreover, the thermal stability of the nanocomposites was increased. The T _d enhancement of the nanocomposite was remarkable.     

Effects of surface modification of fumed silica on interfacial structures and mechanical properties of poly(vinyl chloride) composites

Poly(vinyl chloride) (PVC)-based composites were prepared by blending PVC with nano-SiO₂ particles, which were treated with dimethyl dichlorosilane (DMCS), γ-methylacryloxypropyl trimethoxy silane (KH570). The dispersion and interfacial compatibility of nano-SiO₂ particles in PVC matrix was characterized by SEM, which indicated that DDS had a better dispersion and compatibility than UTS but worse than KHS. The mechanical properties, processability and effective interfacial interaction of nano-SiO₂/PVC composites were studied. The nano-SiO₂ particles treated with KH570 or DMCS significantly reinforced and toughened the PVC composites. The maximum impact strength of PVC composites was achieved at a weight ratio of nano-SiO₂/PVC:4/100. The tensile yield stress increased with increasing the content of treated inorganic particles. The incorporation of untreated nano-SiO₂ particles adversely affected the tensile strength of the composite. Although the equilibrium torques of all nano-SiO₂/PVC composites were higher than that of pure PVC, the surface treatments did reduce the equilibrium torque. The interfacial interaction parameter, B, and interfacial immobility parameter, b, calculated respectively from tensile yield stress and loss module of nano-SiO₂/PVC composites, were employed to quantitatively characterize the effective interfacial interaction between the nano-SiO₂ particles and PVC matrix. It was demonstrated that the nano-SiO₂ particles treated with KH570 had stronger effective interface interaction with PVC matrix than those treated with DMCS, which also had stronger effective interface interaction than the untreated nano-SiO₂ particles.     

Dye-sensitized nanocrystalline solar cells based on composite polymer electrolytes containing fumed silica nanoparticles

We report remarkably high energy conversion efficiency (4.5% at 100 mW cm(-2)) of a dye-sensitized solar cell in the solid state, using composite polymer electrolytes containing fumed silica nanoparticles.     

Mesoporous silica reinforced by silica nanoparticles to enhance mechanical performance

Silica nanoparticle/mesoporous silica composite films were prepared by direct mixing with mechanical stirring and thermal imidization. The structural morphology was elucidated by scanning electron microscopy and the surface of the film was imaged by atomic force microscopy. The functional groups and desorption process of the films were elucidated by Fourier transform infrared spectroscopy and thermal desorption spectroscopy. The mechanical properties were investigated using a nanoindenter system. The gel matrix and the filler are very compatible because they have similar molecular content. The composite films had a higher mechanical strength than pure porous silica film. Their strength is related to the silica nanoparticle content. The interfacial compatibility, dispersion effect, and interfacial strength also affect the mechanical strength of composite films.     

Synthesis and properties of water-soluble silica nanoparticles

Methods for synthesis and optimum conditions of the formation of stable water-soluble silica nanoparticles are presented. The silica nanoparticles were synthesized by the hydrolytic polycondensation of tetraethoxysilane using two methods: under alkaline conditions (Stöber´s method) or in an acetic acid medium followed by the modification by grafting triethylene oxide moieties on the particle surface. The structure of the modified silica nanoparticles was confirmed by the data of IR and NMR spectroscopy. Polydispersity was evaluated by gel permeation chromatography and dynamic light scattering. The formation and stability of Langmuir monolayers of the silica nanoparticles modified by triethylene oxide moieties were studied.     

Effect of ZnO nanoparticles on the thermal and mechanical properties of epoxy-based nanocomposite

Effect of ZnO nanoparticles particles on the mechanical properties and the curing behavior of an epoxy nanocomposite were studied. Nanocomposites were prepared using different loadings of pre-dispersed ZnO nanoparticles having an average size of 40 nm. The surface topography and morphology of the nanocomposites were studied using atomic force microscope (AFM). The mechanical properties of nanocomposites were studied using analytical techniques including dynamic mechanical thermal analysis and micro-Vickers hardness. Effects of ZnO nanoparticles on the curing behavior of these nanocomposites were investigated utilizing isothermal and non-isothermal differential scanning calorimeter techniques. In addition, chemical compositions of coatings containing different ZnO nanoparticles contents were studied using a Fourier transform inferred. It was found that, ZnO nanoparticles can effectively influence the mechanical properties of epoxy coating. In addition, lower curing degrees, and therefore crosslinking density of epoxy coating including higher ZnO nanoparticles were obtained. This effect was completely different at low and high loadings of the particles.