Papers by Keyword: Thermal Stability

Abstract: In this study, an eco-friendly coconut oil-based polyol blend was synthesized for bio-based waterborne polyurethane (WBPU) and WBPU-silane composite coatings. It was demonstrated that an increase in silane content incorporated into the WBPU matrix significantly enhanced the corrosion protection of WBPU coatings. Results also show a fourfold increase in the adhesion strength of WBPU-silane composite coatings as compared to that of bare WBPU coatings. Further, the water contact angle revealed that hydrophobic properties increase as the silane content incorporated into the WBPU matrix increases. This work provides a novel route for enhanced corrosion protection utilizing a bio-based polyol blend.

Abstract: B-type carbonated hydroxyapatite (C_BHA) is potentially an excellent biodegradable bioceramic for bone repair. However, conventional sintering results in formation of undesired phases. Therefore, microwave sintering of C_BHA was investigated to assess the possibility to reduce formation of unwanted phases. Pellets with 0.8 mol% of B-type carbonate were sintered in a multimode instrumented cavity under static air with short thermal cycles. They were prepared from a C_BHA powder alone and from a mixture of C_BHA and carbon powder to generate a local in-situ CO₂ atmosphere. XRD, FT-IR, SEM and BET analyses indicated that C_BHA densification with increase temperature lead to decomposition into apatite. The addition of carbon powder to the C_BHA that generate a CO₂-rich atmosphere around the samples did not prevent the decomposition. Efficient control of temperature and atmosphere composition is required to improve microwave sintering of C_BHA bioceramics.

Abstract: Microcrystalline cellulose (MCC) has been widely used in the production of composite materials because it is inexpensive, easy to process, good mechanical properties and environmentally friendly. Despite its advantages, MCC has disadvantages such as poor thermal stability, hygroscopic and poor compatibility with hydrophobic materials. Understanding the thermal behavior of MCC is important because thermal degradation occurs at different rates and directly affects the final product. In this study, the MCC/ SiO₂ hybrid materials were prepared using in-situ sol-gel synthesis, followed by the investigation of their thermal stability and degradation kinetics using thermogravimetric analysis (TGA). Degradation kinetics were analysed using two model-free analysis (i.e. Flynn-Wall-Ozawa, FWO and modified Coats-Redfern, CRm) to evaluate the degradation behaviour (conversion degree (α) of 0.1 to 0.8) and activation energies (E_a) of MCC, MCC/ sol-gel silica (MCC/SiO₂) and modified MCC (mMCC/SiO₂) at heating rates (β) of 10, 20, 30 and 40 °C/min. Thermal stability results showed that the presence of silica on MCC had no influence on the degradation temperature of the hybrid material however, it slightly shifted the T_onset to higher values. The presence of silica also increased the final residue of the hybrid, especially in mMCC/SiO2 samples. DTG curves clearly show that all samples exhibited one step degradation process. The kinetics study assumed that all samples has single reaction mechanism as the fitted line was parallel in almost all conversion degrees (α) in both FWO and CRm methods. E_a calculated for MCC, MCC/SiO₂ and mMCC/SiO₂ are in good fit with both FWO and CRm model where the R² observed more than 0.97. E_a was increased in both methods, MCC/SiO₂ and mMCC/SiO₂ as compared to MCC, which implied that the addition of sol-gel silica to MCC could promote a stepwise degradation.

Abstract: One of the ways to enhance thermal stability and reduce the flammability of polymers is the introduction of fillers with flame retardant properties. The paper studies the effect of natural zeolites of the Sakhaptinsk and Shivyrtuysk deposits on the thermal stability and flame retardancy of epoxy composites. The thermal stability of epoxy composites was characterized by thermogravimetric analysis in oxidizing (air) and inert (argon) atmospheres. The parameters of thermo-oxidative degradation and thermal degradation of the samples with a filler concentration of 0, 1, 5, and 10 wt% were studied. Flame retardancy of epoxy composites filled with zeolite was evaluated using the limiting oxygen index. The limiting oxygen index increases with increasing concentration of natural zeolites in the epoxy matrix. The study revealed some differences in the first and second stages of heating the epoxy composites depending on zeolite type and did not reveal significant difference in the thermal behavior of the epoxy composites in the third stage of the heating. Thermal stability and flame retardant properties are more dependent on the zeolite content in the epoxy matrix than zeolite type.

Abstract: AbstractEpoxy syntactic foams (SF) filled with hollow glass microspheres (HGM) were prepared by simple resin casting method and characterization in this study. The effect of varying the amount of HGM on the specific mechanical and water absorption properties of SF composites were investigated. Five different composition of SF (SFT60-0.5 to SFT60-2.5) were compared with the neat epoxy matrix. The wall thickness of the microballoons differ because of its different percentile size distribution (10^th, 50^th and 90^th), which reflects in its density variation. The results show that the specific tensile and flexural strength increases with an increasing filler (HGM) content. The density of SF filled with HGM reduces with increasing volume fraction of filler content. Scanning electron microscopy was done on the failed samples to examine the fractured surfaces. The water absorption capacity of the SF was also investigated as it relates to the HGM volume fraction variation. All the syntactic foam composition shows a better diffusion coefficient capacity than the neat epoxy resin. This makes it applicable in structural purposes and several marine application products such as Autonomous Ultimately Vehicle (AUV).

Abstract: The article shows that synthetic wollastonite, obtained on the basis of a mixture of calcium oxide and silicon oxide, as well as calcium carbonate and silicon dioxide, increases the thermal stability of epoxy materials filled with it. The chemical and atmospheric resistance of epoxy films filled with wollastonite is on average 0.2 – 0.3% lower than that of the base composition, which is probably due to the porosity of both synthetic and natural wollastonite. The phase composition of synthetic wollastonite does not significantly affect chemical resistance of epoxy materials filled with it. The chemical resistance of epoxy compositions, regardless of their composition, is lower in weakly acidic solutions and water, in comparison with weakly alkaline and saline solutions.

Abstract: The present paper is aimed to investigate the effect of modifiers, such as poly (phenylene sulfone) and poly (aril ether ketone), on physico-mechanical and thermophysical properties of heat-resistant epoxy-anhydride binder based on EHD epoxy resin. It was found that the addition of poly (phenylene sulfone) and poly (aril ether ketone) can improve mechanical and impact strength characteristics of the binder with no reduction in heat resistance and thermal stability.

Abstract: According to the results of thermodynamic calculations, it has been determined that the usage of alloys with Hf content near to its maximum possible value solubility in the copper matrix is not appropriate. It is more appropriate to use alloy compositions with lower Hf content. With the help of calculations and analysis of experimental data, it has been established that in order to ensure the thermal stability of SPD-formed structures it is sufficient to obtain a concentration of dissolved in copper matrix Hf of about 0.01 at.%. It has also been shown that the average grain size formed by the SPD is a determining factor in the strength properties of these alloys; an increasing Hf concentration in the copper matrix is not always a condition for higher hardness values. However, higher concentrations of dissolved hafnium in the copper matrix will determine the higher mechanical characteristics of precipitation hardening of the alloys after heat treatment.

Abstract: The effects of rare earth (RE) elements (La, Y) addition on thermal stability and corrosion behavior of Mg₆₈Zn₂₈Ca₄ amorphous alloys were investigated in this paper. The investigated Mg-Zn-Ca-RE amorphous alloys exhibit good thermal stability and enhanced corrosion resistance. The enhanced corrosion resistance of the RE-containing amorphous alloys is owing to the enrichment of the Zn and RE elements in the oxide layer. The corrosion resistance is further improved with the increasing of RE content.

Abstract: The oxidation processes for compact and powdery samples of titanium, copper, and molybdenum with different volume structure and dispersivity were studied using thermal analysis, electron microscopy, and X-ray diffraction. It is established that producing of metals with a modified structure under conditions of high-energy impact (severe plastic deformation, electric explosion of a thin wire) in accordance with intermediate annealing leads to an increase in the content of oxygen in the form of solid solutions and oxides; the oxide component’s share, form and localization within the material depend on physicochemical properties of both metal and oxide . It is shown that the structural-phase transformations of the oxide component during heating of fine-grained metals and powders have a significant effect on the parameters of the oxidation process of such materials. The thermally induced effects in the oxygen-containing components might play a critical role for the structure stability during long-term use of such materials under cyclic thermomechanical impacts.