Impact, mechanical initiation

The phenomenon of impact or mechanical initiation is really thermal in nature. This was first proposed by Bowden and Yoffe (Refs. 2 and 3), who postulated that any of several mechanical mechanisms could produce heat at tiny local areas and thus raise the local temperature to the ignition point of an explosive. They referred to these tiny locally heated areas as hot spots. The mechanisms that they proposed were ... 

To analyze the particle capture efficiency of the upper lung, particles of different diameters were initially released with a uniform distribution at the trachea inlet and their trajectories were analyzed. Figure 36 shows the locations at which the particles of different size were captured. It is seen that the relatively large 30-pm particles are mainly deposited on the carina by impaction mechanisms. On the other hand, the 10-nm particles have a more uniform distribution pattern. These small particles are deposited mainly by the diffusion process on the entire passage surface. Very few 1-pm particles are captured by the first lung bifurcation, since for this size range. 

Returning to the mechanisms shown in Schemes 14 and 15, it is important to note that the nucleophilic character of the anion of a Bronsted acid not only impacts the initiation step, but also plays a role in the subsequent respective propagation steps. In the latter steps, the lower the nucleophilic character of the anion, the higher the electrophilic character (i.e., reactivity) of the corresponding carbocation, 47, and secondary and tertiary onium ions 48-50 that are formed. 

Disinfectants in potable water, such as chlorine and chlorine dioxide, can impact the performance of plastic piping system components. In this paper, field failures of plastic piping components exposed to potable water are examined. Analysis of the oxidative and mechanical initiation and propagation mechanisms leading to failure is performed. Field and laboratory samples are also compared to assess the test methodologies that are available to evaluate the impact of disinfectants on plastic piping systems. 

Decomposition Hazards. The main causes of unintended decompositions of organic peroxides are heat energy from heating sources and mechanical shock, eg, impact or friction. In addition, certain contaminants, ie, metal salts, amines, acids, and bases, initiate or accelerate organic peroxide decompositions at temperatures at which the peroxide is normally stable. These reactions also Hberate heat, thus further accelerating the decomposition. Commercial products often contain diluents that desensitize neat peroxides to these hazards. Commercial organic peroxide decompositions are low order deflagrations rather than detonations (279). 

Mech nic l Impact Mills. The mechanical types include cmshers, hammer mills, pin disk mills, turbine mills, and mills with air classifiers. Impact Crusher. Feed material is introduced through a feed opening onto a rotor moving at between 25 and 50 m/s (Fig. 11). The initial impact by the rotor causes some size reduction, and the material is accelerated up to the speed of the rotor and flung against the impact plates, where further size reduction occurs. It is possible to wear-protect these units quite well, so that abrasive materials can be handled. The final end particle size can be varied by the inclusion of an outlet grid to vary the residence time in the machine. 

In the absence of fire retardants the material has a limiting oxygen index of 27.5 and may bum slowly. Only some grades will achieve a UL 94 V-1 rating. The Underwriters Laboratories continuous use temperature index is also somewhat low and similar to the polyarylates with ratings of 135-140°C (electrical) and 105°C (mechanical with impact). Initial marketing has emphasised comparisons with the aliphatic nylons for the reasons given in the previous... 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

Fig. 1.85 Oxidation of high-purity iron in oxygen at differing pressures. All figures on curves are in N m . At 1-3 x 10 tol-3 x 10 N/m torr the rate is controlled by the impact of molecular oxygen at I - 3 x 10 N/m torr the initial rate of oxidation is sufficiently high to give void precipitation and the rate decreases with pressure increasing to l-3N/m at pressures greater than this the crack-heal mechanism becomes operative and the rate again increases with pressure (after Hussey and Cohen...

Impact and Friction Initiation of PETN have been extensively studied. This general subject is summarized in Bowden Yoffe (Refs 13 19), Afanas ev Bobolev (Ref 85a), and Vol 7, 135—55, which contains a rather complete description of impact initiation with many refs to the impact and friction initiation of PETN. There is general agreement that impact and friction initiation proceed via a hot spot mechanism (see Vol 7, HI 70—75). There is, however, considerable doubt and controversy about the mechanisms of hot spot formation... 

Mechanical treatment alone may be sufficient to induce significant decomposition such processes are termed mechanochemical or tribo-chemical reactions and the topic has been reviewed [385,386]. In some brittle crystalline solids, for example sodium and lead azides [387], fracture can result in some chemical change of the substance. An extreme case of such behaviour is detonation by impact [232,388]. Fox [389] has provided evidence of a fracture initiation mechanism in the explosions of lead and thallium azide crystals, rather than the participation of a liquid or gas phase intermediate. The processes occurring in solids during the action of powerful shock waves have been reviewed by Dremin and Breusov [390]. 

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