جستجوی مقالات مرتبط با کلیدواژه « پایداری گرمایی » در نشریات گروه « مهندسی شیمی، نفت و پلیمر »

Epoxy resin is used as a wind turbine, coating, adhesive and base material for composites and is widely used in automotive, electronics, and construction industries. Modification of the surface of epoxy nanocomposites in the presence of silica nanoparticles, due to increased adhesion and covalent bonding, increases the impact strength and Also, by adding silica nanoparticles to epoxy resin, the Young's modulus increases linearly, which is due to the increase in nanoparticle dispersion and reduced stress concentration. By adding silica nanoparticles to epoxy resin, the maximum heat flow temperature decreases and the curing rate increases, and also causes the heat flow curve of epoxy nanocomposite to be wider. The presence of silica nanoparticles reduces the activation energy and silica nanoparticles act as a catalyst in the curing reaction with epoxy resin. Two important factors in reducing the thermal degradation of epoxy nanocomposites are proper dispersion of nanoparticles and non-agglomeration. Silica nanoparticles increase the maximum temperature of thermal degradation, thermal stability and the percentage of residual char. In this research, the modeling of curing kinetics of epoxy nanocomposites on morphology, rheological and mechanical properties, activation energy, curing degree, heat flow and investigation of thermal stability and thermal degradation has been reported.

Aromatic polyamides are well known as a main group of high performance and heat-resistant polymers. One of the drawbacks to utilize these polymers is the difficulty in processing due to their insoluble nature in aprotic organic solvents in addition to their high melting point or glass transition temperature. One way of overcoming the main problem of heat-resistant polymers - i.e., enhancing solubility without too much scarifying of the thermal stability is designing new monomers.

Firstly, bis(4-oxybenzoic acid)-1,5-anthraquinone (DA1) and bis(3-oxybenzoic acid)-1,5-anthraquinone (DA2) were prepared through aromatic nucleophilic substitution reaction of 4-hydroxybenzoic acid and 3-hydroxybenzoic acid with 1,5-dichloro anthraquinone, respectively. In the next step, the Yamazaki method was applied for synthesis of novel polyamides by polycondensation reaction of the obtained new diacids with commercial aromatic diamines such as oxydianiline (ODA), p-phenylene diamine (PPDA), 2,6-diaminopyridine (DAP), and diaminodiphenyl methane (DADPM) in presence of triphenylphosphite and pyridine as the activating agents and N-methyl-2-pyrolidone (NMP) as a solvent.

The structures of prepared novel monomers and polymers were characterized using different spectroscopy methods. The thermal and physical properties of novel polymers such as thermal stability and behavior, solubility, viscosity and ultra violet absorption were studied and the structure-property relationship of these polymers was investigated. The prepared polymers showed defined UV-Vis absorption bands at the range of 344-370 nm. Inclusion of an aromatic and bulky anthraquinone unit to the main chain of polymers led to high thermal stability while their solubility was improved in polar aprotic solvents.

Hypothesis: We investigated the effect of thermally-reduced graphene (TRG) nanosheets on electrical conductivity, dielectric constant, electromagnetic interference shielding performance, rheological behavior and thermal stability of polypropylene/polyethylene terephthalate (PP/PET) blend. For this purpose, 50/50 PP/PET blends were prepared through melt compounding in presence of different volume fractions of TRG. The direct current (DC) conductivity, the AC electrical conductivity and EMI shielding effectiveness of composites were measured. The morphology of blends was examined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Findings: The morphology of the samples was co-continuous, and preferential localization of the nanoparticles led to a double percolated structure. This structure enhanced electrical conductivity of the samples considerably. The rheological analysis indicated that a percolated network was formed at low volume fractions of TRG. At 0.1 vol% loading, the conductivity of the composites satisfies the antistatic criterion (10−6 S/m) for thin films. At 2 vol% of graphene, a high electrical conductivity of0.16 S/m was achieved which was considered sufficient for electronic device applications. The dielectric constant and the electromagnetic interference shielding efficiency (EMI SE) of the blends significantly increased with TRG addition. By incorporating 2 vol% of TRG, the dielectric constant increased from 4 (for neat sample) to 9×107 at 10 Hz and the EMI SE increased from 1 dB (for neat sample) to 42 dB at 10 GHz, satisfying the target value for commercial applications. Thermogravimetric analysis (TGA) indicated that addition of TRG effectively enhanced the thermal stability of the samples. Incorporation of TRG not only increased the initial decomposition temperatures but also decreased the rate of decomposition. The enhanced thermal stability of the composites was attributed to the high aspect ratio of TRGs, which served as a barrier and prevented the emission of gaseous molecules during thermal degradation.

A hydrogel was prepared with high swelling capacity and a stable three-dimensional structure under environmental conditions. The methodology in this study involved the design and construction of a three-dimensional network for a superabsorbent hydrogel using hydrolyzed sulfonated polyacrylamide as polymer and nonahydrate aluminum nitrate as crosslinker. Further methodology involved data analysis to achieve a hydrogel optimized by response surface methodology. The optimum hydrogel in terms of three responses (i.e., gelation time, syneresis and swelling) was identified by designing a series of experiments. The chemistry and morphology of optimal superabsorbent hydrogel was determined by bottle tests (swelling and syneresis), rheology, energy dispersive spectroscopy (EDS), and thermogravimetric analysis (TGA). The results of this study demonstrated its polymer concentration of 40,000 ppm and crosslinker concentration of 6 wt% for preparation of optimum hydrogel. The syneresis of the optimum hydrogel was more than 180 days and it swelled to 2800-times of its initial dry weight; its elastic modulus was 15240 Pa with thermal stability by 325°C. The highest swelling rate (4000-times of the dry weight) was observed for a hydrogel with a polyacrylamide concentration of 20000 ppm and a weight ratio of 6 wt% of crosslinker to polymer. An undesirable syneresis time of less than 10 days was obtained. Moreover, the main factor in controlling the gelation time and syneresis was the weight ratio of crosslinker to polymer, while for controlling the swelling capacity it was found to be polymer concentration.

Polyhedral oligomeric silsesquioxane (POSS) - polymer composite materials have been extensively studied in recent years, as they possess nanoscopic structures and functional properties that are not typically seen in conventional hybrid materials. POSS nanoparticles are 1–3 nm in size, monodisperse and rigid. Incorporation of POSS nanoparticles into both thermoplastic and thermoset polymeric matrices by chemical cross-linking or physical blending methods provides excellent reinforcement. In this review, first, we introduce the various structures of polyhedral oligomeric silsesquioxane and then highlight studies on POSS- polymer nanocomposites with an emphasis on enhancements in mechanical, thermal stability and glass transition. The properties of POSS-containing polymer nanocomposites vigorously depend on the amount of POSS and the state of the POSS dispersion (which depends on the surface functional group of POSS). Incorporation of POSS through chemical cross-linking into polymer can influence its structureand improve mechanical properties by reinforcement.The incorporation of POSS nanoparticles into polymer through physical blending relies on favorable surface interactions between POSS and polymer. POSS having surface functional groups that have favorable surface interactions can disperse uniformly in the polymeric matrix. Uniform dispersion of POSS in polymeric matrices helps to improve physical properties of the nanocomposites. Because of the excellent properties, POSS containing polymer nanocomposites are found in diverse area such as tissue engineering and biomedicines.

Glass fiber/phenolic composites are effectively used as heat shields for fabrication of cooled combustion chambers. To improve the performance of these composites as well as to lower their costs, the use of nanosized silicon dioxide (SiO2) particles has been adopted. In this work, we investigated the effect of nanosilica on properties, heat stability and ablation properties of glass fiber/phenolic composites. Glass fiber/phenolic/nanosilica composites were made of 1, 2 and 3 wt% of well-dispersed silica nanoparticals in a phenolic resin. The average size of silica nanoparticals was 45 nm. These nanocomposites were prepared by hot-press process. Ablation properties of the composites were studied by oxy-acetylene torch environment and their mass and linear erosion rates were evaluated. The thermal stability of the produced composite structures was examined by means of thermal gravimetric analysis in air with dynamic scans at a heating rate of 10°C/min from room temperature to 800°C. Bulk samples, of about 20 ±1 mg each, were tested. The results indicated that nano-SiO2 increased the thermal stability of nanocomposites so that their linear and mass erosion rates after modifying with 3 wt% nanosilica dropped by 58% and 53%, respectively. The 3-point bending test results showed that bending strength and modulus of the as-received composites increased with percentage nanosilica added up to 2 wt%, beyond which they decreased. For the glass fiber/phenolic composites with 2 wt% nanosilica particles, embedded in the matrix, strength and modulus increased about 40% and 19%, respectively, in comparison to those for neat glass/phenolic composite.

Nano-fillers have displayed excellent mechanical properties and are widely used in different polymeric matrices for high performance applications. Recently، epoxy resins modified by nano-reinforcing fillers such as multi-walled carbon nanotubes (MWCNTs) and nanoalumina (Al2O3) have been developed for adhesive applications. In this work، the influence of 1. 5 weight percent of various nanofillers namely nanoalumina، MWCNT، nanosilica (SiO2) and talc on the thermal stability، strength adhesion and morphology of diglycidyl ether of bisphenol A (DGEBA) /epoxy novolac adhesives was studied. Thermal stability and degradation، adhesion strength and morphology were measured by the thermogravimetry analysis (TGA)، lap shear strength test and scanning electron and transmission electron microscopic techniques، respectively. The results showed that incorporation of the nanoalumina and MWCNT into the DGEBA/epoxy novolac adhesives increased the lap shear strength. Moreover، the thermal stability of the epoxy adhesive in terms of onset of degradation temperature and char yield (after 800°C) was improved to some extent. By addition of one and half weight percent nanoalumina and/or MWCNTs in the epoxy adhesives، the lap shear strength increased by about 70 and 25 percent، respectively. Among the investigated fillers، nanoalumina demonstrated the best performance in terms of improvements in the lap shear strength، thermal stability and degradation of the epoxy adhesives. When a combination of nanoalumina and MWCNTs reinforcing fillers (0. 75 weight percent nanoalumina and 0. 75 weight percent MWCNTs) was used as a hybrid filler in the epoxy adhesive، a synergism effect on the char yield was observed.

Phenolic resin composites reinforced with short carbon fiber are one of the most usable materials in ultra-high-temperature applications such as thermal protective in aerospace industries. In this work، novolac type of phenolic resin matrix was modified with graphite nanoparticles to prepare multi-layered nanocomposites. The effect of graphite nanoparticles was studied on the thermal stability، ablation and mechanical properties of novolac/short carbon fiber composites to achieve nanocomposite with optimum properties for ultra-high-temperature applications. In order to evaluate thermal stability and ablation properties of composite and nanocomposites، a sample containing 40 wt% short carbon fiber was prepared as a reference and the structure of its polymeric matrix was modified with nanographite particles. The amounts of nanographite powders in nanocomposite samples were chosen as 6، 9 and 12 wt%. XRD Spectroscopy was used to study and investigate the dispersion of the graphite nanoparticles and morphology in the polymeric matrix. The compression molding under hot press method was used to fabricate the composite and nanocomposite specimens. Thermal properties of the nanocomposites were studied by TGA and oxy-acetylene flame test. Three-point bending and wear tests were performed to measure the mechanical and wear properties of the nanocomposites. The obtained results showed that the addition of nanographite improved the thermal stability، decreased the rate of degradation and at the same time decreased the weight loss and ablation rate of the nanocomposites. Addition of 12 wt% nanographite particles increased thermal stability by about 12% compared to the reference sample. Moreover in nanocomposite with 12 wt% graphite، the rate of ablation decreased by more than 19% compared to the reference composite.

In recent years، waterborne polyurethanes as in coatings and adhesives formulations have attracted considerable attention because they are non-toxic، non-flammable and friendly to environment. Besides environmental management، the flexibility، low temperature property، high tensile strength، good adhesion and improved rheological property are specific properties of waterborne polyurethanes. Also low production cost of water borne polyurethanes over solvent-borne polyurethanes is also a reason for their applications. However، these materials have some defects such as weak water resistance and low adhesion in the moisture environment due to sensitivity of their hydrophilic ionic bonds، ether groups، urethane and ester groups to hydrolysis which need to be improved. Also، low heat resistance of these materials is due to a relatively low crystalline melting point or glass transition temperature of hard segments. One of the ways to solve this problem and improve its properties for different applications is the addition of inorganic fillers especially nano-sized layered silicates within polyurethane matrix. In this way water resistance، heat resistance، mechanical properties and modulus increase simultaneously. In this research، waterborne polyurethane nanocomposites with PTMG polyol، IPDI، DMPA (internal emulsifier)، TEA (neutralizer) and 1، 3 and 5weight % of Cloisite 30B as reinforcement were synthesized and characterized. Polarity of the samples was investigated by contact angle test and dispersion of nano particles in the samples was characterized by X-Ray and TEM، Thermal properties and dynamic mechanical properties were measured by TGA and DMTA، respectively. The results showed that incorporation of clay into polyurethanes did reduce water absorption and increased heat resistance، modulus، particle size and contact angle. In recent years، waterborne polyurethanes including coatings and adhesives have attracted considerable attention because they are non-toxic، non-flammable and friendly to environment. Besides environmental management، flexibility، low temperature property، high tensile strength good adhesion and improved rheological property are specific evidence of waterborne polyurethanes. Also low production cost of water borne polyurethane over solvent-borne polyurethanes is also a reason for their applications. However، these materials have some defects such as weak water resistance and low adhesion in the moisture environment due to sensitivity of their hydrophilic ioned bonds، ether groups، urethane groups and ester groups to hydrolysis which are needed to be improved. Also، low heat resistance of these materials is due to relatively low crystalline melting point or glass transition temperature of hard segments. One of the ways to solve this problem and improve its properties for different application is the addition of inorganic fillers especially nanosized layered silicates within polyurethane matrix. In this way water resistance، heat resistance، mechanical properties and modulus increase. In this research، waterborne polyurethane nanocomposites with PTMG polyol، IPDI، DMPA (internal emulsifier)، TEA (neutralizer) and 1، 3 and 5weight % of cloisite 30B as reinforcement were synthesized and characterized. Polarity of the samples was investigated by Contact angle test، dispersion of nano particles in the samples were characterized by X-Ray and TEM، Thermal properties and dynamic mechanical properties were measured by TGA and DMTA respectively. Results showed that incorporation of clay into polyurethane samples caused the reduction of water absorption، increasing of heat resistance، modulus، particle size and contact angle.

polyacrylamides are water soluble macromolecules. These polymers are widely used for flocculation, separation and treatment of solid-liquid phase materials. In this research, organic-inorganic hybrid of polyacrylamide/silica nanoparticle is prepared via radical polymerization. First, the silica nanoparticle surfaces were modified by 3-methacryloxypropyltrimethoxysilane as coupling agent using a sol-gel technique in aqueous media in acidic condition. Afterwards, the modified nanoparticles are copolymerized by acrylamide monomer in presence of a peroxide initiator during a free radical polymerization. The chemical structure of the prepared modified nano-silica as well as polyacrylamide nanocomposite was studied and confirmed by FTIR spectroscopy technique. The morphology of nanocomposite was investigated by scanning electron microscopy. The SEM micrograph showed that the surface of the composite did not display any phase separation. Nanoparticles distribution was investigated by SEM-EDX technique. The results showed a uniform distribution of particles throughout the polymer bulk. TEM analysis showed the presence of silica nanoparticles in bulk of polymer which is an indicative of suitable dispersion of nanoparticles. The thermal stability of hybrid nanocomposite with that of polyacrylamide was compared by TGA technique. The higher thermal stability of hybrid nanocomposite with respect to homopolymer is indicative of a reaction between the modified nanoparticles and polyacrylamide chain. The presence of silicaparticles in copolymer was also confirmed with EDX analysis in ash content of hybrid nanocomposite.

Ablative materials play a strategic role in aerospace industry. These materialsproduce a thermal protection system which protects the structure، theaerodynamic surfaces and the payload of vehicles and probes duringhypersonic flight through a planetary atmosphere. In this work، we investigatedthe effect of refractory zirconium oxide on mechanical، heat stability and ablationproperties of asbestos/phenolic/zirconia composites. The asbestos/phenolic/zirconiacomposites were produced with different percentages of zirconia filler from 7 to 21%with average size of 7 μm and different number of layers of asbestos، say 3 to 6layers. These ablative composites were made by an autoclave curing cycle process. The densities of the composites were in the range of 1. 68 to 1. 88 g/cm3. Ablation properties of composites were determined by oxy-acetylene torch environment and burn-through time، erosion rates and back surface temperature in the first required 20 seconds. Thermal stability of the produced materials was estimated by means of thermal gravimetric analysis، in both air and nitrogen which consisted of dynamic scans at a heating rate of 10°C/min from 30 to 1000°C with bulk samples of about 20±1 mg. The results showed that when the amount of zirconia was raised from 7% to 21%، the erosion rate and the back surface temperature of composites increased by about 24% and 26% respectively، and the heat capacity of the composites increased byabout 85%. Also، the result showed that when the thickness of composites of 4. 2 mm was increased to 10. 1mm the burn-through time raised by about 226% and erosion rate dropped by about 41%. These composites displayed the maximum flexural strength when the amount of zirconia was about 14%.