Fibers specific heat values

Fig. 14. The variation of the specific heat jumps at glass-transition temperatures of elacc-epoxy composites, versus the fiber volume content, uf. The values for the factor X and the mesophase, (uj and matrix, (nm) volume fractions, versus uf, as derived from the values of the respective AC, s are also plotted...

In addition to wool, other hygroscopic textile materials such as cotton and linen underwent a threefold increase in their specific heat at constant vapor pressure. The relatively high specific heats derived from equations in the study, which are considered to represent those incurred in actual use of the hygroscopic textiles, explain the well-known buffering action of these fabrics toward sudden changes in indoor or outdoor temperatures (2l). A compilation of the specific heat of a variety of textile fibers at 20-200°C indicates that considerable variation in the values of this thermophysical property occurs with different fibers (e.g., a value of 0.157 for glass and 0.1 9 cal/g.°C for Nylon 66 are reported), and that additional research is needed to establish the extent to which specific heat affects the characteristics of thermal transmission in textiles (22). 

The heat capacity of a few fibers of interest has been measured. The values listed in Table 8.11 show that the specific heat does not vary much from fiber to fiber. 

The coefficient of linear thermal expansion, specific heat, thermal diffusivity, thermal conductivity for 1-D and 2-D SiC/RBSN at four temperatures in nitrogen measured parallel and perpendicular to the fibers are summarizedin Tables III and IV. In general, through-the thickness thermal conductivity value at room temperature for SiC/RBSN composites is low when compared with a value 7 W/m-k for the unreinforced RBSN or with a value of 30 W/m-k for the sintered silicon nitrides [13]. Both weak bonding between the SiC... 

The density p, file specific heat Cp and file thomal cnductivity k are conqmted as proper avoages of file single resin and fiber jn tqiaty values. 

The thermal degradation behavior of OPF in comparison with flax and hemp fibers was studied [39]. They reported that OPF is thermally more stable comparing to hemp and flax fibers as its first decomposition peak occurs at 301.71 °C, whereas for flax it was 286.66°C and for hemp, it was only 243.18°C. They have also reported the specific heat capacity of OPF at varying temperatures in the range of 20°C to 150°C in comparison to that of flax fiber and hemp fiber. The specific heat capacity of OPF varied from 1.083 J/g°C to 3.317 J/g°C when the temperature increased from 20°C to 150°C. This trend was similar to that exhibited by flax fiber and hemp fiber. However the specific heat capacity values of OPF is less than the other two fibers at all temperatures, whereas... 

The specific heat of amorphous plastics increases with temperature in an approximately linear fashion below and above Tg, but a steplike change occurs near the Tg. No such stepping occurs widi crystalline types. The high degree of die molecular order for crystalline TPs makes their values tend to be twice those of the amorphous types. The TSs has the highest values. To increase TC the usual approach is to add metallic fillers, glass fibers, foamed structure, or electrically insulating fillers such as alumina. 

Acryhc fibers discolor and decompose rather than melting when heated, but they have very good color and heat stabihty at temperatures less than I20°C. In a study by American Cyanamid (using Federal Test Specification TT-P-I4Ia. Method 425.2) the yeUowness of acryhc fiber was measured as a function of temperature. Compared to a value of 0.0 for a pure white body, the original fiber had a yeUowness index of 0.04—0.10. After 30 minutes of exposure at II5°C the yeUowness increased only slightly to 0.II—0.17. After 6 h at I30°C, however, the yeUowness increased to 0.38—0.41. 

The heating of the fiber up to 1000 - 1200 °C decreased the value of specific surface because of sintering of oxide particles. The fibrous alumina-based materials annealed above 1400 °C become more sintered, their porosity did not exceed 30 %, the specific surface decreased down to 5 m2/g, and the grain sizes were 100-150 nm. At the same time, the strength of this material was enhanced. 

In the oxidation stage, the PAN fiber will increase in density from 1.18 gcm to about 1.36-1.38 gcm for the oxidized PAN fiber (opf). The actual density will depend on whether the final product is to be used as opf or is required for onward processing to carbon fiber. The final density of an opf product will depend on the opf product specification, whereas for carbon fiber, the density must be at least 1.36 gcm , as otherwise, the fiber will tend to pull apart and break on entering the LT furnace. The upper opf density limit for the production of carbon fiber varies with the manufacturer and some manufacturers will use a value as high as 1.40 gcm , but others claim that this would produce inferior carbon fiber. The residual exothermicity of SAP heated in air at 230°C (Figure 5.8) after a 3 h treatment, has some 35% exothermic heat remaining in the oxidized fiber. 

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