Thermal stability of CoNiPtCuAu nanoalloys: from segregation to melting properties.

Preparation and thermal properties of quaternary mixed nitrate with low melting point

A novel pentaerythritol stearate ester‐based zinc alkoxide (PSE‐Zn) was synthesized through a two‐step method. Zinc ethoxide was first prepared from zinc acetate and ethanol, and then with ethanol as the solvent, the obtained zinc ethoxide ester was heated together with pentaerythritol stearate to produce PSE‐Zn. PSE‐Zn could be used as a promising poly(vinyl chloride) (PVC) thermal stabilizer. Conductivity tests, thermal aging tests, and UV‐visible spectroscopy were used to study the thermal stabilization performance of the thermal stabilizers of PSE‐Zn on PVC. The results showed that, with a lower melting point and a good compatibility with PVC, PSE‐Zn presented a significant improvement in maintaining the initial color of PVC. However, the color of PVC stabilized with pure PSE‐Zn began to degrade when the sample was heated at 180°C for 60 min indicating the unsatisfactory long‐term thermal stability presented by PSE‐Zn on PVC. So, some commercial thermal stabilizers, for example, calcium stearate (CaSt2), zinc stearate (ZnSt2), or CaSt2/ZnSt2were employed along with PSE‐Zn to further improve the stabilizing performance. The results showed that there was a synergistic effect that was between 2 phr PSE‐Zn and 2 phr CaSt2and made the stabilized PVC possess the best thermal stability. Because of the presence of free hydroxyl in PSE‐Zn, which could chelate ZnCl2, PSE‐Zn can then restrain “zinc‐burning” phenomenon when PSE‐Zn and ZnSt2were used together. It was also found that PVC stabilized with 3 phr PSE‐Zn and 1 phr CaSt2/ZnSt2had the best color stability. The mechanism of thermal stability of PSE‐Zn was also discussed. As a metal alkoxide, PSE‐Zn is readily to neutralize HCl. In addition, PSE‐Zn is able to replace the allyl chloride to terminate the dehydrochlorination of PVC. Moreover, because of the stearate radicals, PSE‐Zn had a very low melting point, a better compatibility with PVC, and a lubrication action. With these excellent properties, PSE‐Zn would be able to make PVC achieve an excellent initial color and a long‐term stability. J. VINYL ADDIT. TECHNOL., 24:314–323, 2018. © 2017 Society of Plastics Engineers

Nontoxic and efficient coal gangue thermal stabilizer (Y‐CG) was prepared by cheap and readily available calcium oxide, coal gangue (CG) and stearic acid(SA). The proposed mechanism of Y‐CG thermal stability was studied by XRD, FT‐IR, BET and thermal stability test. The results show that when the mass ratio of CG to water was 1:7, the loading of Ca(OH)2 between CG sheets reached the maximum value of 3.01 mmol/g. The hydrophobic property of the Y‐CG surface was better when modified at 75°C for 2 h with 5% SA. With 10% Y‐CG, the Congo red test showed thermal stabilization time of PVC improved up to 55.9 min and the torque rheological test showed dynamic thermal stabilization time of PVC improved up to 62.10 min. The TG curves show that Y‐CG had little effect on the onset degradation temperature of PVC. Still, Y‐CG can absorb HCl and initiate the carbonization of PVC to inhibit degradation. Compared with the commonly used thermal stabilizers, the thermal stability of Y‐CG was only slightly worse than that of hydrotalcite thermal stabilizers. The early reaction was that the strong base Ca(OH)2 in Y‐CG adsorbs HCl, and the CaCl2 was stably loaded in the CG layered structure, which avoided the adverse effects caused by strongly alkaline. The late reaction was that SA and Ca(OH)2 form weakly alkaline calcium stearate to absorb HCl. At the same time, calcium stearate can also be used as a compatibilizer to improve the compatibility between Y‐CG and PVC.

A novel thermal stabilizer for rigid poly (vinyl chloride) has been examined. Its high stabilizing efficiency is detected by the extent of discoloration and the mass loss rate of the degraded samples when compared with some of reference stabilizers. A higher stabilizing efficiency is produced as the novel thermal stabilizer is mixed with different co-stabilizers. The study results for thermo oxidative stability of the degraded samples with novel thermal stabilizer have shown that the novel thermal stabilizer can prevent or delay the thermal oxidative degradation of PVC. A probable stabilization mechanism for the novel thermal stabilizer has been proposed.

In this study, an attempt is made to investigate the rheological behavior and thermo-rheological properties of PVC in the presence of new tannin based thermal stabilizer. Tannin based epoxy resin has been used as nonmetallic thermal stabilizer additive to study its effects on the rheological behavior of PVC. Frequency sweep tests were applied for monitoring and evaluating the rheological properties of PVC formulations with different levels from this derivative (0, 5, 10, and 20) parts per hundred ratio (phr) using a parallel plate rheometer at 165 °C. The general results indicate that the adding of tannin epoxy resin to PVC can provide enhanced dynamic thermal and process stability and improved the rheological properties. In these formulation systems, the rheological properties of PVC with tannin epoxy resin were more closed to that of PVC without plasticizer so that the plasticization effect of tannin epoxy resin was at very limited level. The rheological properties, such as storage modulus, complex viscosity and damping factor proved simultaneously that the PVC stabilized with tannin derivative exhibits excellent rheological behavior and melt thermal stability properties which were very comparable to that of PVC stabilized with 2 phr Reapak B-NT/7060; a commercial type thermal stabilizer.

In this paper, the melting behaviors of Rh–Ag–Au nanoalloys are investigated with MD simulation. For Rh–Ag–Au nanoalloys, icosahedron structure was considered. The local optimizations of Rh–Ag–Au nanoalloys were carried out with the BH algorithm. The interatomic interactions were modeled with the Gupta potential. The local optimization results of Rh–Ag–Au nanoalloys show that Au and Ag atoms prefer to locate on the surface, and Rh atoms prefer to locate in the inner shells. The bond order parameter result is compatible with the excess energy analysis. It is noted that structures with more Ag–Au bonds are more energetically stable. Caloric curve, heat capacity, Lindemann index, and RMSD methods were used for estimating the melting temperatures of Rh–Ag–Au nanoalloys. According to the simulation results, melting temperatures depend on the composition. Also, it is discovered that nanoalloys are generally melting in two stages. Surface melting of the third shell is occupied by Ag and Au atoms, and then homogeneous melting of the inner shells is occupied by Rh atoms. It is found that the difference between surface melting temperatures and homogeneous melting temperatures in Ag-poor compositions is more significant than that of Ag-rich nanoalloys. In addition, the melting temperatures of the nanoalloys are found to be increased as the size of nanoalloys increases.

To study the correlation between the thermal and chemical stability of insulin formulations with various insulin hexamer ligands. The thermal stability was investigated using differential scanning calorimetry (DSC) and near-UV circular dichroism (NUV-CD). The formation of chemical degradation products was studied with reversed-phase and size-exclusion chromatography and mass spectrometry. An excellent correlation between the thermal stabilization by ligand binding and the deamidation of Asn(B3) was observed. The correlation between thermal stability and the formation of covalent dimer and other insulin related products was less clear. Zinc was found to specifically increase the deamidation and covalent dimer formation rate when the insulin hexamer was not further stabilized by phenolic ligand. Thiocyanate alone had no effect on the thermal stability of the insulin zinc-hexamer but significantly improved the chemical stability at 37 degrees C. At low temperatures thiocyanate induced a conformational change in the insulin hexamer. NUV-CD thermal scans revealed that this effect decreased with temperature; when the thermal denaturation temperature was reached, the effect was eliminated. Thermal stability can be used to predict the rate of Asn(B3) deamidation in human insulin. Chemical degradation processes that do not rely on the structural stability of the protein do not necessarily correlate to the thermal stability.

Platinumnanowires (NWs) have been reported to be catalyticallyactive toward the oxygen reduction reaction (ORR). The edge modificationof Pt NWs with metals M (M = Au,Ag, or Pd) may have a positive impact on the overall ORR activityby facilitating diffusion of adsorbed oxygen, Oads, andhydroxyl groups, OHads, between the {001} and {111} terraces.In the present study, we have employed classical molecular dynamicssimulations to investigate the segregation behavior of Au, Ag, andPd decorating the edges of Pt NWs. We observe that, under vacuum conditions,Pd prefers to diffuse toward the core rather than stay on the NW surface.Ag and Au atoms are mobile at temperatures as low as 900 K; they remainon the surface but do not appear to be preferentially more stableat edge sites. To effect segregation of Au and Ag atoms toward theedge, we propose annealing in the presence of different reactive gasenvironments. Overall, our study suggests potential experimental stepsrequired for the synthesis of Pt nanowires and nanoparticles withimproved Oads and OHads interfacet diffusionrates and consequently an improved ORR activity.

Optimal design, characterization, and thermal stability of bio-based Ca/Na/Zn composite stabilizer derived from myrcene for poly(vinyl chloride)

With the aim to understand the effect of bio-polyphenols and calcium-based thermal stabilizer on the rheological and thermal degradation properties of polyvinyl chloride (PVC), this paper practically investigates the rheological and thermal degradation behaviors of PVC using melt rheology method. Thermal degradation study was carried out by monitoring the complex viscosity of PVC using a parallel-plate rheometer run by time sweep mode at 215 °C. The presence of calcium into tannin structure after modification allows the later to have HCl scavenger ability which is considered as an essential property of most PVC thermal stabilizers. The thermal stability monitoring by time sweep rheology measurements was further corroborated by dynamic rheological data, indicating that the increase in loading value of tannin–calcium into PVC from 1 to 3 part per hundred of PVC resin resulted in a notable enhancement of the PVC storage modulus and complex viscosity due to the increased thermal stability, stable chemical structure and a higher level of polymeric chain entanglements. The rheological parameters obtained by frequency sweep at 165 °C in terms of storage and loss moduli and damping factor proved simultaneously that the PVC formulations with tannin–calcium exhibit an obvious improvement in the rheological properties which were comparable to that of PVC stabilized with Reapak B-NT/7060, an industrial-type thermal stabilizer. The results obtained indicated a promising correlation between the thermal stabilization performance of tannin–calcium as a new and fully bio-based thermal stabilizer and the rheological properties of PVC.

A new di-mannitol adipate ester-based zinc metal alkoxide (DMAE-Zn) was synthesized as a bi-functional poly(vinyl chloride) (PVC) thermal stabilizer for the first time. The materials were characterized with Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). Characterization results confirmed the formation of Zn–O bonds in DMAE-Zn, and confirmed that DMAE-Zn had a high decomposition temperature and a low melting point. The thermal stability of DMAE-Zn on PVC also was tested by a conductivity test, a thermal aging test, and a UV-visible spectroscopy (UV-VIS) test. PVC stabilized by DMAE-Zn had a good initial color and excellent long-term stability. UV-VIS also showed that the conjugated structure in PVC stabilized by DMAE-Zn was almost all of the triene, suggesting that the addition of DMAE-Zn would suppress the formation of conjugated structures above tetraene. The dynamic processing performance of PVC samples tested by torque rheometer indicated that, having a good compatibility with PVC chains in the amorphous regions, DMAE-Zn contributed a good plasticizing effect to PVC. DMAE-Zn thus effectively demonstrates bi-functional roles, e.g., thermal stabilizers and plasticizers to PVC. Furthermore, FT-IR, a HCl absorption capacity test, and a complex ZnCl2 test were also used to verify the thermal stability mechanism of DMAE-Zn for PVC.

Due to their excellent thermophysical properties and high stability, inorganic salts and Forsalt mixtures are considered promising thermal energy storage materials for applications operating at high temperatures. A mixture of binary salts, such as CaCl2 (58 wt.%)-LiCl (42 wt.%), was investigated in this work to understand their thermal properties and stability for use in TES systems. Thermophysical properties, such as onset melting and crystallization temperature, enthalpy of fusion, and crystallization enthalpy, were all investigated experimentally via the use of a simultaneous thermal analyzer. The experimental findings demonstrated a suitable onset melting temperature of 488 °C and a solidification temperature of 480 °C. The heat of fusion was observed as 206 J/g, whereas the heat of crystallization was recorded as 180 J/g. Thermal repeatability tests indicated little variations in melting temperature; however, fusion enthalpies changed significantly over the course of 30 heating-cooling cycles. Additionally, the results obtained from the thermogravimetric analysis showed relatively weak thermal stability with considerable mass changes. This might be caused by the salts decomposing at elevated temperatures. In order to validate this, a high-temperature in-situ X-ray diffraction technique was used to verify the thermal instability of the binary salt mixture with and without thermal cycling. The thermal decomposition of parent salts and the subsequent formation of new phases with the formation of voids were shown to be the cause of thermal instability. It is concluded that the binary mixture of chloride salt showed suitable thermal properties but relatively weak thermal stability, which may limit its use in practical applications.

Calorimetric evaluation of phase change materials for use as thermal interface materials

Xanthan gum is the staple viscosifier and fluid loss control agent used in reservoir drilling and completion fluids. In over 40 years of use in this application xanthan has built up a deserved reputation for reliable performance in drilling and completion fluids, and for generally minimizing damage to reservoir formations. Its only peculiarity is its low transition or melting temperature (Tm) in low salinity fluids and in bromide brines, meaning that its viscosity can collapse very suddenly at 70-120°C. This can be a nuisance in applications where viscosity maintenance at high temperatures is important, but a benefit if the application requires a self-breaking polymer. Previous research has shown that formate brines are capable of increasing the thermal stability of xanthan gum by increasing its Tm and by providing anti-oxidant protection. The extent to which the thermal stability of xanthan may be increased depends primarily on the type and concentration of the alkali metal formate salt in solution. The most effective brine from this perspective is a very concentrated potassium formate brine, which can increase the Tm of xanthan to over 200°C (396°F) and raise the 16-hour thermal stability to almost 180°C (356°F). The objective of the study described in this paper was to investigate if it was possible to use combinations of commercial polymer stabilizing additives to further increase the temperature ceiling of xanthan gum dissolved in formate brines. The opportunity was also taken to look at the high-temperature behavior of xanthan in rubidium formate brine and cesium acetate brine. Before starting on the additive screening program the thermal stability of xanthan in formate brine was compared against other well-known natural biopolymers used in drilling and completion fluids. The tests confirmed that xanthan really was the best performing viscosifier in buffered and alkaline formate brines, although welan showed some promise in unbuffered formate brines at neutral pH. Tests also showed that there was no significant difference in thermal stability between six different commercial xanthan products in formate brines. The transition temperature of xanthan in cesium formate brine was found to reach a maximum of almost 180°C (356°F) at a brine density of 2.20 g/cm3 (18.4 lb/gal) compared with almost 200°C (396°F) in 1.57 g/cm3 (13.1 lb/gal) potassium formate brine, and blends of the two brines produced Tm values in the range 180-200°C (356-396°F). High-density rubidium formate brines raised the xanthan Tm to around 185°C (365°F) at a density of 2.11 g/cm3 (17.6 lb/gal). Hot-rolling tests of cesium acetate brines viscosified with xanthan have shown that cesium acetate is able to stabilize xanthan to similar temperatures as cesium formate. The best additive package for increasing the thermal stability of xanthan in potassium formate brine was found to be a blend of magnesium oxide and 5% v/v of a polyethylene glycol. Potassium formate brine containing xanthan and this additive package was found to retain some viscosity after hot rolling for 16 hours at at least 194°C (381°F). It seems likely that the glycol acted as a sacrificial scavenger, mopping up free radicals before they could attack the xanthan. This finding validates a suggestion from work carried out in 1980’s that the best thermal stabilizer package for xanthan would include a concentrated salt brine containing a glycol and an oxygen scavenger Deployment of the stabilizer package identified in this study should raise the thermal stability ceiling of xanthan in buffered formate-based non-damaging drilling and completion fluids by more than 20°C (36°F).

Geared towards reinforcing thermoplastics of high melting points with nanocellulose, this study evaluated the factors affecting the thermal properties of and the thermal stabilizing effect of acetylation on nanocellulose with different average degree of polymerization (DPv) from bacterial cellulose (BC). Cellulose nanocrystals with DPv values of 300 and 500 were prepared by hydrolyzing BC nanofibers with a DPv of 1100 using hydrochloric acid. The thermal stability decreased after acid hydrolysis and showed a decreasing trend with decreasing DPv. The decrease in thermal stability is attributed to the increase in the number of reducing ends (REs) with decreasing DPv. Heterogeneous acetylation to an average degree of substitution of 0.38 improved the thermal stability, and the degree of improvement increased with decreasing DPv. The dependence of the degree of improvement on the DPv is attributed to possible protection of the REs by more stable acetyl groups. The influence of protecting the REs on the degree of improvement in thermal stability was further confirmed by sodium borohydride (NaBH4) reduction. The findings suggest that the thermal stabilization caused by acetylation to nanocellulose with small DPv is a combined effect of protecting both the surface OH and the REs; while for nanocellulose with high DPv, the thermal stabilization caused by acetylation is mainly due to protection of the surface OH.