Half-decomposition temperature

Figure 18.9. Predicted glass transition temperature (thin line) and half decomposition temperature (thick line) of amorphous random copolymers of styrene and oxytrimethylene, as functions of the composition.

Initial decompositiom temperature. Half decomposition temperature. Char yield at 600 C. 

The thermal stability of the (alkylsulfonyl)methyl-substituted polymers was evaluated from dynamic thermogravimetric analysis (TGA) under N2 (Table II). All the (alkylsulfonyl)methyl-substituted polymers were stable to 300 C. Figure 3 shows that all of the (alkylsulfonyl)methyl-substituted polymers have similar decomposition behavior their initial decomposition and half decomposition temperatures were very close. Table III shows that the side chain length does not affect the thermal decomposition behavior. Backbone structure does affect the decomposition behavior slightly. MST shows a two-step decomposition and MSEE shows the highest thermal stability (Figure 4) of the three (methylsulfonyl)methyl-substituted polymers. 

Van Krevelen [12] found a reasonably linear correlation between the half-decomposition temperature T 1/2 and the... 

TABLE 54.2. Half-decomposition temperature T1/2 and monomer yield for selected polymers. 

Solvent polarity also affects the rate of peroxide decomposition. Most peroxides decompose faster in more polar or polari2able solvents. This is tme even if the peroxide is not generally susceptible to higher order decomposition reactions. This phenomenon is illustrated by various half-life data for tert-huty peroxypivalate [927-07-1]. The 10-h half-life temperature for tert-huty peroxypivalate varies from 62°C in decane (nonpolar) to 55°C in ben2ene (polari2able) and 53°C in methanol (polar). 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Useful constants that can be measured in the course of the experiment are time of full foaming, tf and time of CBA half-decomposition, t1/2. Both values fall exponentially with temperature. 

Traditionally, diazonium tetrafluoroborates are decomposed neat in the solid state. This solid, placed in a flask with large outlets and which must not be more than half full of the salt, is gently heated near its surface until decomposition starts. Often no more heat is required, the decomposition continuing spontaneously with evolution of dense vapors of boron trifluoride. The reaction medium is often brought to dull redness and the fluorinated product distills if sufficiently volatile.1,3 The filled reaction flask can also be immersed in a fluid brought to ca. 20 to 50 C above the decomposition temperature of the diazonium salt, previously determined in a capillary tube.1,3,200,201 In another procedure, the reaction flask can be heated to this temperature while empty, then the diazonium tetrafluoroborate is added little by little 200-201 This latter method has been adapted to perform the decomposition of diazonium tetrafluoroborates in a continuous way by two techniques ... 

Half-Life. Once these activation parameters have been determined lor a initiator, half-life times tit a given temperature, i.e.. the lime required for 50 decomposition at a selected temperature, and half-life temperatures for a given period, i.e.. the temperature required for 509f decomposition of an initiator over a given time, can be calculated. Half life data arc useful for comparing the activity or one initiator with another when the half-life data arc determined in the same solvent and at the same concentration and. preferably, when the initiators are of the same class. 

Initiators (1) and (2) have 10-h half-life temperatures of 237°C and 201°C, respectively. It has been reported that, unlike organic peroxides and aliphatic azo compounds, carbon—carbon initiators (1) and (2) undergo endothermic decompositions (62). These carbon—carbon initiators are useful commercially as fire-retardant synergists in fire-resistant expandable polystyrenes (63). 

The heat resistance of a polymer may be characterised by its temperatures of "initial" and of "half" decomposition. The latter quantity is determined by the chemical structure of the polymer and can be estimated by means of an additive quantity the molar thermal decomposition function. The amount of char formed on pyrolysis can be estimated by means of another additive quantity the molar char-forming tendency. 

In Fig. 21.1 the dissociation energy of the weakest bond of the same polymers, supplemented with the data of a number of radical initiators (peroxides and azo compounds) is plotted against the most characteristic index of the heat resistance, viz. the temperature of "half decomposition" (Ta4/2). The relationship is evident, though not sufficiently accurate for a reliable estimate of T yi-... 

FIG. 21.1 Correlation between temperatures of half decomposition and dissociation energy of weakest bond. 

Besides the reaction rate k previously defined in Section 3.2, which provides an instantaneous description of how fast a certain process is at a given temperature, some integral parameters (within a time range) were defined for the same purpose. One such parameter is the half decomposition time tio, which is the time required to get W/Wo = 1/2. Making the approximation that k does not vary with the heating time (extremely short TRT or very high values for E ), the formula for v2 calculated from rel. (7) Section 3.2 is the following ... 

Td 1/2 Temperature of half decomposition (weight loss), in degrees Kelvin. 

Van Krevelen [11] developed a correlation for the temperature of half decomposition Tdj/2 which he defined as "the temperature at which the loss of weight during pyrolysis (at a constant rate of temperature rise) reaches 50% of its final value". In this correlation, which is given by Equation 16.1, Td 1/2 is approximated in terms of the ratio of the molar thermal decomposition function Ydl/2 divided by the molecular weight M per repeat unit. The observed values of Td,i/2 [11] are listed in Table 16.1 for a number of polymers. 

The ASTM Committee E27.02 (Thermal Stability/Condensed Phases) has also recently published a document (44) entitled "Standard Practice for Calculation of Hazard Potential Figures of Merit for Thermally Unstable Materials". This includes calculations of 1) Time to thermal runaway, 2) critical half thickness, 3) critical temperature, and 4) adiabatic decomposition temperature rise. 

---