Elevated temperature stability

The only Ot-alloy of commercial importance is Ti—5A1—2.5Sn. This alloy is weldable, has good elevated temperature stability and good oxidation resistance to about 600°C, and is used for forgings and sheet-metal parts, such as aircraft engine compressor cases because of its weldability. The ELI version of this alloy is used in the cryogenic area of rocket engines. However, the alloy is difficult to produce and has not been designed into more recent aerospace systems. 

Chemical stability predictions are sometimes complicated by the difficulty of determining the pH value of suspensions, which often changes because of surface coating of electrodes and differences between bulk-suspension and supernatant-vehicle readings. Accelerated elevated temperature stability testing often has a pronounced adverse effect on viscosity, particle solubility, and size distribution. 

Polyphenylquinoxaline (PPQ) This resin is supplied by NASA Langley as monoether polyphenylquinoxaline in a solvent mixture 1 1 of practical grade m-cresol and mixed xylenes. The solid content is about 16.6% based upon final polymer weight. PPQ has demonstrated excellent elevated temperature stability and represents a different family of polymers for evaluation in this program. The chemistry of this polymer is shown below. 

Effect of Water Vapour at Elevated Temperatures—Stability cfX and Y Zeolites When exposed to steam at 350°C, sodium X zeolite loses structure and adsorption character. The partial... 

Carbon-Fiber Composites. Cured laminates of phenolic resins and carbon-fiber reinforcement provide superior flammability resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 14 shows the dependence of flexural strength and modulus on phenolic-carbon-fiber composites at 30-40% phenolic resin (112). These composites also exhibit long-term elevated temperature stability up to 230°C. 

Brishois, E.J., Handa, H., Major, T.C., Bartlett, R.H., Meyerhoff, M.E., 2013. Long-term nitric oxide release and elevated temperature stability with 5-nitroso-A-acetylpenidllamine (SNAP)-doped Elast-eon E2As polymer. Biomaterials 34, 6957-6966. 

Applications. Alloy Ti-5522-S is a semicommercial product for high-temperature applications in the range of425 to 540 °C (800 to 1000 °F). The alloy was developed for high creep strength and elevated-temperature stability in applications such as jet-engine components. 

Beta III Elevated temperature stability for bar, sheet and plate... 

Phenolic Resin - Thermosetting resin with elevated temperature stability, e.g. phenol formaldehyde. May be blended with other polymers, or used as an adhesive. Water-proof versions used for wood bonding. 

Sodium terbium borate is used in solid-state devices. The oxide has potential application as an activator for green phosphors used in color TV tubes. It can be used with Zr02 as a crystal stabilizer of fuel cells which operate at elevated temperature. Few other uses have been found. 

Hthiated 4-substituted-2-methylthia2oles (171) at -78 C (Scheme 80). Crossover experiments at—78 and 25°C using thiazoles bearing different substituents (R = Me, Ph) proved that at low temperature the lithioderivatives (172 and 173) do not exchange H/Li and that the product ratios (175/176) observed are the result of independent metala-tion of the 2-methyl and the C-5 positions in a kinetically controlled process (444). At elevated temperatures the thermodynamic acidities prevail and the resonance stabilized benzyl-type anion (Scheme 81) becomes more abundant, so that in fine the kinetic lithio derivative is 173, whereas the thermodynamic derivative is 172. 

Conjugation of the newly formed double bond with the carbonyl group stabilizes the a p unsaturated aldehyde provides the driving force for the dehydration and controls Its regioselectivity Dehydration can be effected by heating the aldol with acid or base Normally if the a p unsaturated aldehyde is the desired product all that is done is to carry out the base catalyzed aldol addition reaction at elevated temperature Under these conditions once the aldol addition product is formed it rapidly loses water to form the a p unsaturated aldehyde... 

Health nd Safety Factors. Although propargyl alcohol is stable, violent reactions can occur in the presence of contaminants, particularly at elevated temperatures. Heating in undiluted form with bases or strong acids should be avoided. Weak acids have been used to stabilize propargyl alcohol prior to distillation. Since its flash point is low, the usual precautions against ignition of vapors should be observed. 

The procedures used for estimating the service life of solid rocket and gun propulsion systems include physical and chemical tests after storage at elevated temperatures under simulated field conditions, modeling and simulation of propellant strains and bond tine characteristics, measurements of stabilizer content, periodic surveillance tests of systems received after storage in the field, and extrapolation of the service life from the detailed data obtained (21—33). 

The long-term stability of the nonaqueous coating films under elevated temperature and moderate humidity is reported to be better than aqueous coatings (30). Furthermore, color resolution and sensitivity of reacted nonaqueous coating films ate excellent. 

Crystalline CaHPO 2H20 loses both water molecules in a single step at moderately elevated temperature or upon storage to yield the anhydrous salt. The presence of free moisture accelerates this dehydration, which results in anhydrous dicalcium phosphate, often as a hard mass. Addition of a few percent of tetrasodium pyrophosphate or trimagnesium phosphate, Mg2(P0 2> stabilizes the dihydrate. The mechanism, however, is not well understood. Nonetheless, these materials are used widely to stabilize CaHPO 2H20, particulady for toothpaste appHcations. 

Dimensional Stability. Plastics, ia general, are subject to dimensional change at elevated temperature. One important change is the expansion of plastics with increa sing temperature, a process that is also reversible. However, the coefficient of thermal expansion (GTE), measured according to ASTM E831, frequendy is not linear with temperature and may vary depending on the direction in which the sample is tested, that is, samples may not be isotropic (Eig. 7). 

Many antioxidants ia these classes are volatile to some extent at elevated temperatures and almost all antioxidants are readily extracted from their vulcanizates by the proper solvent. These disadvantages have become more pronounced as performance requirements for mbber products have been iacreased. Higher operating temperatures and the need for improved oxidation resistance under conditions of repeated extraction have accelerated the search for new techniques for polymer stabilization. Carpet backiag, seals, gaskets, and hose are some examples where high temperatures and/or solvent extraction can combine to deplete a mbber product of its antioxidant and thus lead to its oxidative deterioration faster (38,40). 

Many organic hahdes, especially alkyl bromides and iodides, react direcdy with tin metal at elevated temperatures (>150° C). Methyl chloride reacts with molten tin metal, giving good yields of dimethyl tin dichloride, which is an important intermediate in the manufacture of dimethyl tin-ha sed PVC stabilizers. The presence of catalytic metallic impurities, eg, copper and zinc, is necessary to achieve optimum yields (108) ... 

Stability. Ascorbic acid, a white crystalline compound, is very soluble ia water and has a sharp, acidic taste. In solution, the vitamin oxidizes on exposure to air, light, and elevated temperatures. Solutions of ascorbic acid turn yellowish, followed by development of a tan color. Ascorbic acid is stable to air when dry but gradually darkens on exposure to light. 

Aqueous solutions of vitamin > 2 at pH 4.0 to 7.0 show no decomposition during extended storage at 25°C. For optimum stability at elevated temperatures, solutions should be adjusted to pH 4.0 to 4.5. Aqueous solutions in this pH range maybe autoclaved for 20 min at 120°C without significant decomp o sition. 

When used alone at low temperatures, diaLkyl thiodipropionates are rather weak antioxidants. However, synergistic mixtures with hindered phenols are highly effective at elevated temperatures and are used extensively to stabilize polyolefins, ABS, impact polystyrene (IPS), and other plastics. 

Mechanical Properties and Stability at Elevated Temperature. One increasingly important characteristic of carbon fibers is their excellent performance at elevated temperatures. Strength tested in an inert environment remains constant or slightly increases to temperatures exceeding 2500°C. Amoco s high modulus pitch carbon fiber P-50 maintains approximately 80% of room temperature modulus at temperatures up to 1500°C, then decreases more rapidly to 30% at 2800°C (64). The rapid decrease in modulus is a result of increased atomic mobiHty, increa sing fiber plasticity. 

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