Carbon black free radical

Effects of dose rate on the concentration of free radicals were examined by performing the test at 163 kGy dose with a dose rate of 1 kGy/h, and the results are shown in Figure 25.25(a). Concentration of free radicals decreases with increasing dose rate, i.e. the lower dose rate produced higher concentration of radicals. Decay kinetics of the radiation-induced carbon black free radicals were measured by taking ESR of samples at an interval of 3-5 min after withdrawal of the radiation source. Figure 25.25(b) shows the measured decay... 

Bound rubber in an unvulcanised carbon black-rubber mix. It results from the production of free radicals in the mastication of rubber these radicals attach themselves chemically to the particles of carbon black and form a proportion of carbon gel which is insoluble in the usual rubber solvents. 

Szwarc (99) found a great affinity for methyl radicals in carbon black. Donnet and co-workers [58, 100, 101) determined the concentration of free radicals on carbon black surfaces by the fixation of the radicals of isobutyronitrile, 3,5-dichlorobenzoyl peroxide, and lauroyl peroxide. The number of radicals bound by the surface coincided satisfactorily with the number of unpaired electrons determined by e.s.r. 

Since the number of free radicals calculated from the dilference in nitrogen uptake (1.13 102 /gm) agreed well with the number of unpaired electrons as determined by e.s.r. (0.80-10 /gm), the aroxylic structure seemed very likely. The reaction of oxidized carbon black with styrene can be explained on this basis 102). 

The reaction with free radicals plays an important role in the interaction of carbon black with rubber (103) and with styrene (58, 102). 

The number of free radicals detected by EPR in porous carbons varies from 10 to 10 radicals per gram and is strongly dependent on the per cent carbon content of the carbon. Likewise in the carbonization of organic materials the number of radicals is strongly dependent on the temperature of carbonization a maximum number of radicals is attained by carbonization between 500 and 600°. Heat treatment of carbon blacks formed by pyrolysis of natural gas and oils also results in a variation 182) of the number of unpaired electrons. 

Since carbon black has many stable free radicals, it may be added to polymers such as polyolefins to retard free radical by attracting and absorbing other free radicals. It is customary to add small amounts of other antioxidants to enhance the stabilization by a synergistic effect whereby many antioxidant combinations are more stable than using only one antioxidant. 

Carbon black and many polynuclear hydrocarbons are effective inhibitors of oxidation. Their antioxidant properties are believed to arise from their ability to trap free radicals. 

Light. Ultraviolet (uv) light promotes free-radical oxidation at the mbber surface which produces discoloration and a brittle film of oxidized mbber (35). This skin cracks in random directions to form a pattern called crazing, which can be prevented by the addition of carbon black fillers or uv stabilizers. Black stocks are more resistant to uv light than are gum or light-colored stocks. Nonblack compounds require larger quantities of nonstaining antioxidants which should bloom to the surface as the surface uv stabilizers deplete. 

A large amount of water is usually contained in the dictyo-structure of the hydrated silica precipitated, including both free and combined moisture. The precipitate is separated out by filtering and washed to remove impurities, mainly Na and acid radical ions. The cake is made into slurry again by stirring and the latter is then spray-dried to yield the powdery product of white carbon black. 

Light Stabilizers Pigments (carbon black, iron oxides), UV absorbers (hydroxyphenones, benzotriazoles), excited-state quenchers (organic Ni complexes), free-radical scavengers Hindered amine light stabilizers [piperidines, hindered amine light stabilizers (HALS)]... 

Experimental studies of filled rubbers are complicated by several things, such as the effect of the magnetic susceptibility of the filler, the effect of free radicals present at the surface of carbon black, the complex shape of the decay of the transverse magnetisation relaxation of elastomeric materials due to the complex origin of the relaxation function itself [20, 36, 63-66], and the structural heterogeneity of rubbery materials. 

The presence of free radicals deriving from carbon black could also complicate the interpretation of NMR data in the case of filled rubbers, because radicals may cause a substantial decrease in T2. Two types of radicals have been detected in carbon-black-filled rubbers localised spins attributable to the carbon black and mobile spins deriving from rubbery chains [86]. Mobile spins are formed because of the mechanical breakdown of polymer chains when a rubber is mixed with carbon black. The concentration of mobile spins increases linearly with carbon black loading [79, 87]. 

The amount of radicals in carbon black filled rubbers decreases significantly upon extraction of free rubber with the aid of a solvent containing a free radical scavenger. The extraction nevertheless causes a substantial increase in the fraction of the T2 relaxation component with the decay time of about 0.02-0.03 ms [62], This increase is apparently caused by an increase in the total rubber-carbon black interfacial area per volume unit of the rubber due to the removal of free rubber. The T2 relaxation component with a short decay time is also observed in poly(dimethyl siloxane) (PDMS) filled with fumed silicas [88], whose particles contain a minor amount of paramagnetic impurities. Apparently, free radicals hardly influence the interpretation of NMR data obtained for carbon-black rubbers in any drastic way [62, 79]. 

Finally, carbon black did reduce burning rate somewhat, especially when high structure grades were used (Table X), but not sufficiently to act as a sole flame-retardant—only as an assistant to more conventional flame-retardant additives. The importance of char and free radicals in burning mechanisms (10) and the relationship of char and free radicals to carbon black should prompt further exploratory studies of this property—viz., carbon black s synergistic combination with conventional flame retardants. 

It is interesting to note that conventional carbon black supports promote the formation of peroxide, which then decomposes into radicals that attack the membrane. However, the role of graphitized carbon materials (such as CNTs) in peroxide formation is less clear. Smalley suggested that the curvy graphitic structure of CNTs deactivates free radicals by stabilizing them through enhanced delocalization. It would be worthwhile to determine whether the formation and fate of peroxide is any different between the carbon black and the CNT. At any rate, it is well known that the rate of formation of peroxide is greatly reduced by elimination of the carbon black support. Evidence of this is clear from the work we have done on carbonless electrodes (PTFE-bonded Pt black electrodes) and those with a hybrid structure. - " ... 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

---