Continuous Use Temperature Rating

Over the years many attempts have been made to provide some measure of the maximum service temperature which a material will be able to withstand without thermal degradation rendering it unfit for service. Quite clearly any figure will depend on the time the material is likely to be exposed to elevated temperatures. One assessment that is being increasingly quoted is the UL 746B Relative Temperature Index Test of the Underwriters Laboratories (previously known as the Continuous Use Temperature Rating or Index). 

The most widely accepted test method is that used by Underwriters Laboratories (UL) to determine the temperature index (continuous-use temperature rating) of a plastic material. This test procedure is predicated on a linear time-temperature relationship for the aging of a thermoplastic polymer, i.e., thermal degradation... 

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

In the absence of fire retardants the material has a limiting oxygen index of 27.5 and may bum slowly. Only some grades will achieve a UL 94 V-1 rating. The Underwriters Laboratories continuous use temperature index is also somewhat low and similar to the polyarylates with ratings of 135-140°C (electrical) and 105°C (mechanical with impact). Initial marketing has emphasised comparisons with the aliphatic nylons for the reasons given in the previous... 

Figure 4. Maximum continuous use temperatures of some engineering thermoplastics based on Underwriters Laboratories ratings...

See also electronic plastic glass transition temperature heat distortion point IEEE thermal ratings continuous-use temperature. 

RTI Relative thermal index (formerly named continuous-use temperature) is the maximum service temperature at which the critical properties of a material will remain within acceptable limits over a long time, as established by UL 746B. There can be up to three independent RTI ratings assigned to a material ... 

Maximum continuous use temperatures are based upon the Underwriters Laboratories (UL) rating for long-term (100,000 hours) continuous use, and specifically on the elevated temperature which causes the ambient temperature tensile strength of the material to fall to half its unexposed initial value following exposure to that elevated temperature for 100,000 hours. The tests provide a continuous use temperature for a plastic in the absence of stresses. The maximum use temperature of PP is compared with other thermoplastics in Table 13. It can be seen that other commodity plastics and some other engineering plastics have a significantly lower maximum continuous use temperature than PP. However, polycarbonate has a higher maximum continuous use temperature in comparison to PP. 

The Arrhenius curve obtained by this procedure can be used to predict the property half-life of the polymer at a given temperature. The UL temperature index (continuous-use temperature) is determined by the interrelationship of the curves for the control and for the composite being indexed. The continuous-use temperature can be determined by dropping a vertical line from the intersection of the 100,000-h line and the Arrhenius curve (see Fig. 3-28). Since most applications are designed with a life safety factor of 2, a part molded from an indexed compound will still meet minimum requirements of the application after 100,000-h exposure at the rated temperature . 

Maximum continuous use temperatures are based upon the Underwriter s Laboratories rating for long-term (100,000 hours) continuous use, and specifically on the elevated temperature that causes... 

Many grades of acetal resins are Hsted in Underwriters Eaboratories (UL) Kecogni d Component Directory. UL assigns temperature index ratings indicating expected continuous-use retention of mechanical and electrical properties. UL also classifies materials on the basis of flammability characteristics homopolymer and copolymer are both classified 94HB. 

Rinse. When transfer of the required volume of regenerating solution to the column has been completed, a small amount of regenerating solution occupies space immediately above the resin bed, between resin particles in the bed, and within the resin particles. It must be displaced with water before the column can be returned to the adsorption step. Rinsing should begin at the same flow rate as used during regeneration and continue at that rate until a volume of water equal to 1—2 bed volumes has been used. After that, the flow rate is increased to the rate normally used during the adsorption step, and continued at that rate until the effluent is of satisfactory quaHty, as deterrnined by pH, conductivity, or resistivity. The water need not be at an elevated temperature unless the process stream is above ambient temperature. 

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

Table 1.8 indicates the service temperatures which can be used with a range of plastics. It may be seen that there are now commercial grades of uiweinforced plastics rated for continuous use at temperatures in excess of 200°C. When glass or carbon fibres are used the service temperatures can approach 300°C. 

Ethyl formate is to be produced from ethanol and formic acid in a continuous flow tubular reactor operated at a constant temperature of 303 K (30°C). The reactants will be fed to the reactor in the proportions 1 mole HCOOH 5 moles C2H5OH at a combined flowrate of 0.0002 m3/s (0.72 m3/h). The reaction will be catalysed by a small amount of sulphuric acid. At the temperature, mole ratio, and catalyst concentration to be used, the rate equation determined from small-scale batch experiments has been found to be ... 

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