Initiator half-life temperature

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. 

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 ... 

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 ... 

In this paper we present a meaningful analysis of the operation of a batch polymerization reactor in its final stages (i.e. high conversion levels) where MWD broadening is relatively unimportant. The ultimate objective is to minimize the residual monomer concentration as fast as possible, using the time-optimal problem formulation. Isothermal as well as nonisothermal policies are derived based on a mathematical model that also takes depropagation into account. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and time is studied. 

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

In Figme 4 is shown the effect of initiator half-life for an initiation activation energy of 120 KJ/mol on the optimum temperature and optimum time. It can be seen that the optimum temperature is almost independent of the half-life. As expected, the optimum time increases with an increase in half-life. Closer study of the results reveals that an almost constant optimal temperature is due to high NL, Values. A much higher temperature would cause to be higher than the desired Mf. 

In this paper we formulated and solved the time optimal problem for a batch reactor in its final stage for isothermal and nonisothermal policies. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and optimum time was studied. It was shown that the optimum isothermal policy was influenced by two factors the equilibrium monomer concentration, and the dead end polymerization caused by the depletion of the initiator. When values determine optimum temperature, a faster initiator or higher initiator concentration should be used to reduce reaction time. 

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. 

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). 

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,... 

Comparative studies on the thermal degradation of polyisoprene, natural rubber (cis-polyisoprene) and gutta percha (t r tns-polyisoprene) all of which have the same chemical composition have shown that they differ in their thermal degradation characteristics. Studies under vacuum at 290-380 "C have shown that the decomposition of natural rubber (NR) is initiated at comparatively low temperatures at a considerable rate, whereas its decomposition rate at higher temperatures (above 330 "C) is to some extent slower than that of gutta percha and polyisoprene (Figure 2.1). The half-life temperature for synthetic polyisoprene is 320 C [1, 2]. 

A list of some peroxo compounds that generate free radicals is given in Table 3.3, extensive information can be found in the literature. The initiators are selected according to their thermal half-lives to ensure that at the polymerization temperature they provide a source of free radicals. The rate equation for the thermal half-life is given by tj/2 = 0.693-kd, where kd is the rate constant for the thermal decomposition. In technical applications one often uses the temperature at which within a certain time interval one half of the initiator is decomposed (e.g., quoted as 10 h half-life temperature). 

The initiator was injected into the second barrel where it mixed with the unmelted polypropylene in the solids conveying zone. In the melting section, a process temperature of about 200°C resulted in an initiator half-life on the order of 5 sec. This is about the residence time in the melting section, so that when the acrylic acid was injected into barrel 4, polypropylene radicals would already have been formed. The grafting reaction of acrylic acid was essentially completed within one barrel section. An atmospheric vent was used in the fifth barrel to remove unreacted acrylic acid volatiles and entrapped air. 

One of the nice features of free-radical polymerization is that values of the preexponential coefficients and activation energies (or alternately half-life values at various temperatures) can be obtained in the literature (such as in Odian (1991)) or from their manufacturers (such as Wako Chemical Corp.) for a variety of initiators, and these numbers do not normally change no matter what the fluid environment the initiator molecules are in. Thus, if we want to decompose more than 99% of the starting initiator material in the reactor, we just have to wait for the reaction to proceed up to five times the initiator half-life. The other attractive feature of free-radical polymerization is that free-radical reactions are well known and radical concentrations can be directly measured. Thus, we know, for example, that if we want to preserve radicals in solution, we should not allow oxygen gas (O2) in our system, because reactive radicals will combine with oxygen gas to form a stable peroxy radical. That is why reaction fluids were bubbled with N2, CO2, Ar, or any inert gas, in order to displace O2 gas that comes from the air. Finally, Iree-radical polymerization is not sensitive to atmospheric or process water, compared to other polymerization kinetic mechanisms. 

First-stage polymerization of styrene in ether (33.4 g styrene, 0.200 g ether, 0.34 g V-65, or AIBN in a 300-ml Parr reactor system) was carried out at 80°C up to five times initiator half-life. Then, the reactor fluid was withdrawn through a 1/8-in. copper tube that is immersed in ice-water bath. The cold reactor fluid was collected into a 1000-ml glass reactor that contains 400 ml distilled water and 12 g acrylic acid (AA). The mixture was continuously mixed at room temperature for at least 2 h in order to soak-in the AA monomer into the polymer-rich domains. Then, the... 

Table 8.2 Decomposition Rate Constants and Half-life Temperatures for Some Initiators...

Most dentures are fabricated firom the heat-cured formulations with the polymerization rate increasing directly with temperature, proportional to the square root of the initiator concentration. The half-life temperature (ti/2, °C) for BPO at 72°C is 10 h (5). The customary curing cycle of the fully mixed powder/liquid blend is about 90 min at 65°C. Post-curing is usually done at 100°C for 60 min so as to produce a more fully cured denture with low porosity. After cooling, the denture is separated from the embedding material, trimmed, and polished. Red fibrous materials and beads of varying translucency are added in small amoimts prior to curing so as to simulate the appearance fo natural oral-tissue. 

Initiator Half-Life. Once these activation parameters have been determined for an initiator, half-life times at a given temperature, ie, the time required... 

Monoperoxycarbonates are similar in thermal stability to -alkyl peroxyben-zoates and can be used in applications where there is concern that benzene will be formed as a by-product of peroxybenzoate initiation. For example, 00- erf-butyl 0-(2-ethylhexyl) monoperoxycarbonate [34443-12-4], with a 10-h half-life temperature of 99°C, can be used in place of ert-butyl peroxybenzoate, with a 10-h half-life temperature of 104°C, for the polymerization of styrene with only slight changes in reaction conditions. 

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