Half-life temperature

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). 

The eight classes of organic peroxides that are produced commercially for use as initiators are Hsted in Table 2. Included are the 10-h half-life temperature ranges (nonpromoted) for the members of each peroxide class. 

Changing the stmcture of R affects the activity of monoperoxycarbonates as previously discussed for peroxyesters. The other cogenerated radical is an alkoxycarbonyloxy radical. The nature of the R group has practically no effect on the reactivity of monoperoxycarbonates having the same 00-tert-55ky group. The 10-h half-life temperature remains at 100°C for almost ah. 00-tert-huty O-alkyl monoperoxycarbonates. 

Diperoxyketals. Some commercially available di(/ f2 -alkylperoxy)ketals and their corresponding 10-h half-life temperatures (deterrnined in dodecane) are hsted in Table 5 (39). Diperoxyketals thermally decompose by cleavage of only one oxygen—oxygen bond initially, usually foUowed by P-scission of the resulting alkoxy radicals (40). For acychc diperoxyketals, P-scission produces an alkyl radical and a peroxyester. 

Diall l Peroxides. Some commercially available diaLkyl peroxides and their corresponding 10-h half-life temperatures in dodecane are Hsted in Table 6 (44). DiaLkyl peroxides initially cleave at the oxygen—oxygen bond to generate alkoxy radical pairs ... 

Table 7 shows that the nature of the alkyl group, whether primary alkyl, secondary alkyl, or cycloalkyl, does not affect the 10-h half-life temperatures of diaLkyl peroxydicarbonates in trichloroethylene (TCE). AH peroxydicarbonates have about the same 10-h half-life temperature in TCE (49—50°C). 

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 ... 

If the T values of Table I are first fitted against EA values, without first fitting with EE(v) values, poor linea correlations result. For example, for tKe one-hour half-life temperatures of reactions 1 and 4, the squares of the correlation coefficients for these linear regression analyses are only 0.51 and 0.55, respectively. 

Organic peroxides have been used to crosslink elastomers and plastics for over 50 years. The organic peroxides utilised by the rubber industry reactvery predictably. Most are stable at room temperature and will decompose based on their half-life temperature curves. They can represent a severe hazard, however, if they are stored or used improperly. These issues are reviewed in detail. 4 refs. USA... 

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. 

The symmetrical azonitriles arc solids with limited solubilities in common solvents. Some commercial aliphatic azo compounds and their 10-h half-life temperatures arc listed in Tabic 3. 

Dialkyl Peroxydicarbonates. Some commercially available dialkyl peroxydicarbonates and their corresponding 10-h half-life temperatures (determined in trichloroethylene solutions) are listed in Table 7 (45). These peroxides are active at low temperatures and initially undergo homolytic cleavage to produce alkoxycarbonyloxy radical pairs that may subsequendy decarboxylate to produce alkoxy radicals ... 

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). 

Cycle development parameters include temperature, airflow rate, humidity, liquid peroxide concentration, liquid peroxide delivery rate, peroxide vapor delivery temperature, and peroxide vapor half-life. Temperature distribution qualification involves the use of temperature sensors located throughout the sterilant delivery line and throughout the enclosure. Vapor distribution qualification uses... 

Polymerization Process. The copolymer of TFE and PVEX monomer may be prepared according to the well known methods for homopolymerization and copolymerization of a fluorinated ethylene (10, 17, 59, 60, 64). The methods may be broadly classified as polymerization in a non-aqueous system and polymerization in an aqueous system. The polymerization temperature is generally from 5 to 100°C depending on the half-life temperature of the initiator. The pressure may be varied from 0 to 30 kg/cm2 to adjust the composition of the copolymer,... 

The models and material property data for predicting fission gas release from heavy metal contamination and failed particles are described in Refs. 2 and 3. These models give the release-rate-to-birthrate ratio (R/B) from contamination and failed particles as a function of chemical element, isotope half-life, temperature, and burnup. In addition, the effect of fuel... 

It is known that, in the absence of oxygen, PE is thermally stable. The thermogravimetric analysis (TGA) of this polymer shows that decomposition of this polymer begins at about 280 °C, and when the temperature is near to 350 °C, its thermal degradation proceeds rapidly with the elimination of considerable quantities of volatile materials. The half-life temperature (i.e., that leading to 50% weight loss on heating for 40-45... 

This is clearly confirmed by the data on the thermal degradation of propylene and PIB under vacnnm (Table 1.3). Comparison of the number of volatile products released at corresponding temperatures has shown that PIB is less thermally stable than PP thus the PP half-life temperature is 387 °C, while that of PIB is 344 "C. 

Monomer formation occurs when free macroradicals decompose via a chain mechanism without hydrogen atom transfer. The half-life temperature for PS is 360 °C. The thermal degradation products consist mainly of monomer, dimer and trimer. 

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