Maximum use temperature

Aqueous hydrogen fluoride of greater than 60% maybe handled in steel up to 38°C, provided velocities are kept low (<0.3 m/s) and iron pickup in the process stream is acceptable. Otherwise, mbber or polytetrafluoroethylene (PTFE) linings are used. For all appHcations, PTFE or PTEE-lined materials are suitable up to the maximum use temperature of 200°C. PTEE is also the material of choice for gasketing. AHoy 20 or Monel is typically used for valve and pump appHcations. Materials unacceptable for use in HE include cast iron, type 400 stainless steel, hardened steels, titanium, glass, and siHcate ceramics. 

The upper use temperature for annealed ware is below the temperature at which the glass begins to soften and flow (about Pa-s or 10 P). The maximum use temperature of tempered ware is even lower, because of the phenomenon of stress release through viscous flow. Glass used to its extreme limit is vulnerable to thermal shock, and tests should be made before adapting final designs to any use. Table 4 Hsts the normal and extreme temperature limits for annealed and tempered glass. These data ate approximate and assume that the product is not subject to stresses from thermal shock. 

Maximum use temperatures exceed 1000°C except for Si02 (>800° C) melting points range from 1450°C (Kovar) to 3660°C (tungsten). 

Fiber type Density, g/cm Strength, GPa Elongation, % Modulus, GPa Maximum use temperature, °C... 

Tliermosets Specific gravity Tensile strength ModiUns of elasticity, tension Impact strength, lzod Maximum use temperature (no load) HDT at 2.54 Ib in -" Chemical resistance s ... 

In general, properties of polyether sulfones are similar to those of polycarbonates, but they can be used at higher temperatures. Figure 12-6 shows the maximum use temperature for several thermoplastics. Aromatic polyether sulfones can be extruded into thin films and foil and injection molded into various objects that need high-temperature stability. 

Fluorocarbons (PTFE, FEP, PVF2) Powder, emulsions Excellent high temperature properties. TFE to 500 F. FEP is easier to mold, but maximum use temperature is 400 F. Nearly inert chemically. Nonflammable. Loading with conductive filler improves creep resistance. Low coefficient of friction. High-temperature cable shielding, gaskets, heat-shrinkable tubing. 

Polyolefins (Polyethylene, Polypropylene) Powder, pellets Tough and chemical resistant. Weak in creep and thermal resistance. Polyethylene maximum use temperature 210 F, polypropylene 260 F. May be injection and extrusion molded, vacuum formed. Low cost. Antistatic sheet and tiles, heat-shrinkable tubing, deicer boots. 

Fig. 3.12).11,12,17 In this way, a material with a higher modulus, higher strength, and less swelling is obtained and the maximum-use temperature is considerably increased. 

Rearrangement of Eq. (4.4) yields a relation that may be used to calculate the maximum use temperature (Tf) for a given lifetime (tf) ... 

The processing techniques used for CMCs can be quite exotic (and expensive), such as chemical vapor infiltration (CVI), or through pyrolysis of polymeric precursors. Their maximum use temperatures are theoretically much higher than most MMCs or PMCs, exceeding 1800°C, although the practical use temperature is often much lower... 

The maximum use temperature is of course important in connection with high-temperature resin systems. 

Silicon nitride is prized for its hardness (9 out of 10 on the Mohr scale), its wear resistance, and its mechanical strength at elevated temperatures. It melts and dissociates into the elements at 1,900 °C, and has a maximum use temperature near 1,800 °C in the absence of oxygen and near 1,500 °C under oxidizing conditions.41 It also has a relatively low density (3.185 g/cm3). Unlike silicon carbide, silicon nitride is an electrical insulator. The bulk material has a relatively good stability to aggressive chemicals. This combination of properties underlies its uses in internal combustion engines and jet engines. 

The half life Tj must be > Ti. The leading orientation parameter B2 is given approximately2for low degrees of polarisation by B2 = l2 (hv/kT)2/5. Useful NO measurements require typically B2(min) = 0.2, giving a maximum useful temperature of T(max) Ihv/k, for which... 

Plastic and thin-wall metal packings deflect with time and temperature and compress. The problem is aggravated in tall bads and where a plastic approaches ite maximum use temperature. The compression raises pressure drops. The rise in pressure drop, however, is generally smaller than that due to ceramics breakage. 

The organic fire retardants are generally better fire retardants, but they lower the maximum use temperature of the material. Frequently, a combination of inorganic fillers and organic retardants is used to achieve a synergistic effect. 

The polyimides are the best developed class of high temperature polymers with exceptional oxidation resistance. They are generally based on the condensation polymerization of aromatic dianhydrides with diamines forming a soluble polyamic acid, followed by intermolecular cyclodehydration to the insoluble polydiimide. Their maximum use temperature is 358 °C. Recently H. Reimschuessel prepared a polyimide... 

Heating above the melting temperature for a short time (about one minute) destroys the ultra-drawability of gel-spun polymers the extended chains then retract to folded chains. This is also the reason that the maximum use temperature is about 40 °C below the melt temperature. 

Perhaps even more important than their structures, most of these polymers are frequently reinforced by glass fibers or even carbon fibers, which contribute tremendously to their properties, and must be considered in any comparison of their practical performance. Reinforcing fibers generally raise modulus 2-4 fold and usually increase breaking strength somewhat. In crystalline plastics, they often raise maximum use temperatures dramatically. 

Maximum Use Temperature. The most frequent requirement for higher engineering performance is retention of properties at higher temperatures. Whereas most commodity thermoplastics soften and distort in boiling water, engineering thermoplastics are most often characterized by their ability to stand... 

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